Okay kiddo, imagine your grandma passes away and she left a will saying her money and house will go to your uncle. But then, your mommy says "wait a minute, I don't think that's fair. Maybe grandma didn't actually mean to leave everything to Uncle, maybe someone tricked her or she wasn't in her right mind when she wrote the will."
That's when a will contest happens, which is basically when someone questions the validity of the will. They can do this by going to court and presenting arguments for why they think the will shouldn't be followed.
Some reasons why someone might contest a will include things like fraud (someone tricked grandma into writing the will), undue influence (someone convinced grandma to write the will in their favor), or lack of capacity (grandma didn't understand what she was doing when she wrote the will).
It can be a complicated and emotional process, but it's important to make sure that grandma's last wishes are carried out fairly and legally.