Okay kiddo, have you ever played with toy cars or dolls? You know how you can pretend that they are doing things but they aren't really doing them in real life? That's kind of like what notional agreement is.
Notional agreement is when we treat a word like it's a certain gender or number even when it's not really that way. So, just like when you pretend your toy car is driving even though it's not really driving, we can pretend a word is a certain gender or number even when it's not really that way.
For example, let's say we're talking about a group of people - some boys and some girls. Even if there are more boys than girls, we might still say "they are happy" instead of "he is happy" because "they" includes both boys and girls, even though "they" is technically plural and "is" is singular.
So notional agreement is like playing pretend with words, making them act like they're a certain gender or number even when they're not really that way in real life.