Assortative mixing is like playing with your friends who are very similar to you. When you play with your friends who are around your age, you have things in common and you play similarly.
Similarly in assortative mixing, people who are similar to each other hang out or associate more often than with people who are different. It's like when you like to play soccer and you have a lot of friends who like to play soccer too, you're more likely to hang out with them and play soccer together.
This happens in real life too. People who have similar interests, lifestyles, and backgrounds often hang out or associate with each other more often than with people who are different. For example, if you're someone who loves watching movies, you're more likely to hang out with people who also enjoy doing the same.
Assortative mixing can sometimes lead to groups becoming more segregated, but it can also help people find new friends and form relationships. It's like when you make a new friend who also likes to play soccer, you add them to your group of soccer-loving friends and everyone can play together.
Overall, assortative mixing is just a fancy way of saying people tend to hang out with others who are similar to them!