The phrase "key part" means an essential or crucial element in a system, process, or situation. It refers to something that is fundamental and indispensable for the proper functioning or success of what it is involved with.
Motion occurs only in key parts of the frame, such as mouths moving when characters speak or body parts moving when action calls for it. (nextprojection.com)
Bellerin assisted one goal and played key part in the first one. (justarsenal.com)
They'll be without three players that have all played key parts in their season so far. (eplindex.com)