A buffer gives project managers a leeway when unforeseen events occur and is often associated with scheduling in project management. A buffer can be temporal, financial or qualitative in nature, i.e. you have additional time, money or people available for difficult project phases. Buffers can be assigned either to the whole project or to individual activities (work packages).
Time buffers (also called lags or slacks) are often used in combination with time constraints in the project to create some flexibility. Activities that you must complete on time and can't have any delays are displayed on the critical path in the project plan (e.g. in a Gantt chart or network diagram).