To imitate is to copy. This is one of the most basic human instincts because it ties to how people learn, first by watching and then by doing. Think of playing Peek-A-Boo with a child and how quickly the child begins to mimic the actions of the game. People's natural desire to imitate is part of how theatre began.

Traditions are things that have been handed down or passed on: ideas, beliefs, customs, stories, etc. How does this tie to theatre? Think of a rehearsed story that is memorized and performed before an audience (even one as small as a family), like an annual 'reading' of the The Night Before Christmas. The need to pass on traditions is part of how theatre began.