Individual rationality is when someone makes choices that they think are the best for themselves.
Imagine you really love candy and your parents give you two choices: either you can have 1 piece of candy now or you can wait 10 minutes and get 2 pieces of candy.
If you choose to wait for 10 minutes for 2 pieces of candy, that's individual rationality because you think having more candy is better for you. But if you choose to eat the 1 piece of candy now, that's still individual rationality because you think having 1 piece of candy right now is better for you than waiting for 10 minutes for more.
Everyone makes choices that they think will help them or make their situation better. That's individual rationality.