When two separate aspects or relationships in a persons life collide: your friends (one "world") become friends with your girlfriend (a separate "world").
Popularized by The Seinfeld episode "the pool guy", in which Georges Costanzas' girlfriend forms her own friendship with Georges' platonic friend Elaine (Causing Georges' worlds to collide).
Kramer: 'Jerry, don't you see? This world here, this is George's sanctuary. If Susan comes into contact with this world, his world's collide. You know what happens then?'
<Kramer puts his hands together and makes explosion sounds that exemplify what happens when there are worlds colliding>
Some people are not permitting grandparents and grandchildren to see each other after months of quarantine, COVIDcing them that is "for their own good," when the reality is that they just don't like their in-laws.