Most basic explanation I can think of: Tanks (and military equipment in general) require fuel, ammo and spare parts to operate optimally. In general nations want to use weapons that require similar ammo types, fuel and some degree of similar parts(so they can be used interchangeably). So while you can, hypothetically operate say 10 different types of tanks, that might mean you're also having to maintain a source of 10 different types of ammo, fuel and spare parts. Most nations tend to operate like one or two types of tanks for this very reason.
Where this relates to the above is basically:
-Ukraine, being an Ex-member of the Soviet Union, inherited a bunch of soviet type tanks when the USSR fell. T-64, T-72, T-80, etc. This generally isn't too bad since Soviet tanks are generally close to each other in what they require to operate.
-With the 2022 invasion, Ukraine captured a lot of hardware from Russia(including tanks) and has been getting a lot of Poland's sizable Soviet tank arsenal(which they also inherited). Poland, whose national motto might as well be "Fuck Russia", is backfilling those tanks with anything they can get their hands on(They just ordered 1000+ South Korean tanks and are having a factory set up in poland to make them faster). Ukraine is happy because they're keeping their tank forces somewhat up to snuff despite taking losses and Poland is happy because they can buy new toys and have their hand-me downs killing Russians by proxy. Everybody wins!
-However with it being difficult to keep finding Soviet type tanks from friendly nations(and Russia not being willing to sell Ukraine any more tanks for some reason), NATO nations have been offering their own to some extent. Germany offering/allowing leopard tanks to be sent to Ukraine, and the US offering Abrams tanks, complicates the process of getting more parts, ammo and spare parts which are generally not compatible with Soviet designs.
For example: Soviet/Russian Tanks are designed around the 125mm tank shell(of which there are many, many types) while NATO tank designs are designed 120mm tank shells(of which there there are also many, many types). Clearly those aren't compatible with each other, but Ukraine, being a Soviet type tank operator has the advantage of being able to use captured Russian Ammo.
I'm using tanks for the sake of simplicity but NATO has been offering a lot of equipment that is what Ukraine needs but is not terribly compatible with the existing soviet type equipment, thus creating complicating supplying and maintaining such equipment. This a notably a problem with artillery because the Ukrainians are getting newer NATO artillery but they don't quite have the ammo stockpiles to use it as much as they like until more of it is supplied(though apparently factories making such shells are running full tilt at this point) and while they have Soviet type artillery to go around, they're also running low on ammo for those as well because they've been using it up.
For some really fun facts, during WW2, the German army had a MASSIVE issue with this kind of stuff, where they had a lot of varied weapons but very little of it was compatible and a lot of it was a pain to fix and maintain. Even better, at one point the German army operated like 50 different types of truck, none of which were compatible with each other, so if they broke, a lot of times they were just abandoned because they couldn't fix them. And take a wild guess how they're moving the fuel, ammo and spare parts for trucks and tanks and so on? Yep, those trucks they can't easily fix. Enjoy thinking about that for a bit. Now imagine you're the guy who needs to make sure all the trucks are in working order so the trucks can deliver the ammo, fuel and spare parts to keep not only the trucks but also the tanks and such working. Welcome to the job of a logistics officer, where you have to keep track of all this shit in an environment where people keep trying to blow up your stuff(those jerks).
There's a famous quote, attributed to Gen. Omar Bradley, that goes. "
Amateurs talk strategy. Professionals talk logistics" for this very reason. Okay, so maybe that wasn't very basic, but logistics is a complex subject that rarely gets talked about outside of military nerds, logistics types and Hearts of Iron players. It's tricky to explain a way that's both succinct and gives a decent picture of why it matters. Also I listen to way too much Perun and that guy LOVES to talk logistics.