Buffer
Beginner
A small booster part that repeats a signal at full strength so it can travel farther.
Novice
A cell whose output simply copies its input with fresh strength. Inserting buffers along a long wire, or in front of many inputs, keeps the signal fast and its edges sharp.
Expert
A non-inverting repeater (BUF_X1 … BUF_X16, or an inverter pair). Inserted by repair_design for slew, capacitance, fanout and long-wire RC limits, and by timing repair for setup; each one needs a free legal site near the right point on the net.
Explained in Placement (Design Flow).