When someone socialises, they spend time with other people for fun or friendship.
Socialising is usually a good thing because it means people are enjoying each other's company.
Examples:
She socialises with her friends at the park.
Do you like to socialise with other kids?
He socialises a lot because he likes to be with people.
The dog socialises with other dogs at the park.
In the evening, the whole family socialises together.
The cat socialises with the other cats in the neighborhood.