A complicated situation is one that's hard to understand or deal with because many things are happening at once.
When we say something complicate, we usually mean it makes a problem worse by adding new and difficult details. This can happen in all sorts of situations - like planning a big project for work, where you have to consider a lot of different factors and timelines, or trying to solve a tricky puzzle that gets harder as you go along.
(transitive) To make complex; to modify so as to make something intricate or difficult.
