This is the 92nd most frequent Japanese word.
じゃ
"じゃ" is a casual contraction of "では," meaning "then," "well," or "so."
In this sentence, 'じゃ' is used as a transitional phrase to imply moving on to the next action.
じゃ、行きましょう。
Well then, let's go.
Here, 'じゃ' is used colloquially as a contraction of the copula 'では' in informal negation.
ここは教室じゃない。
This is not a classroom.
'じゃ' is part of the word 'じゃがいも' (potatoes) and not used as a stand-alone word here.