What’s the meaning of critical path ,s curve gap ,mile stone activity?

The critical path method (CPM) is an algorithm for scheduling a set of project activities.

S-curve describes a sigmoid function, a mathematical function that produces a sigmoid, or "S"-shaped, curve.

a milestone is an event that receives special attention. It is often put at the end of a stage to mark the completion of a work package or phase. Milestones can be put before the end of a phase so that corrective actions can be taken, if problems arise,and the deliverable can be completed on time.