Long-term validation means checking something to make sure it is still true after a long time has passed.
Imagine you make a cake and you want to make sure it will still taste good after a week in the fridge. You taste a little piece of the cake right after it's baked to make sure it's good. That's like a short-term validation. But to do long-term validation, you put the rest of the cake in the fridge for a week, then you take it out and taste it again to make sure it still tastes good.
Long-term validation is important for many things, like medicine or technology. Doctors and scientists need to test drugs to make sure they are safe and effective over a long period of time. If they only tested the drug for a short period and then gave it to people, they might not know about any bad effects that only happen after a long time has passed.
In summary, long-term validation means checking something to make sure it still works or is still true after a long time has passed.