Wiktionary
n. (context idiomatic English) A central or major role in a situation; a position in which one is surrounded by or very involved in complex, changing events.
Usage examples of "thick of things".
The Englishman obviously intended to be in the thick of things after all, whether he liked Dutchmen or not!
Tas called in excitement, climbing up onto the railing and preparing to leap down into the thick of things.
As Jonathan's squire, Alanna was very much in the thick of things, serving at secret meetings and accompanying her Prince to what seemed to be an endless number of parties and dances.
He was a Maquis, after all, and they seemed to have a preference for being in the thick of things.
And your people will have to be right in the thick of things beside ours, handling mind-defense and getting in whatever licks they can.
When Berry hears the clash of steel, he has to gallop into the thick of things.