The critical path is the sequence of activities that determines the minimum possible duration of the project. When viewed in a network diagram, the critical path is the longest path to the end of the project.

If the network diagram contains only Finish to Start relationships, any delay to an activity on the critical path will delay the completion date. If not, it is not true (try with 2 activities, activity A 10 days predecessor with a Start to Start relationship of the activity B 20 days  - it is possible to delay A without to delay the whole project)