Drive strength
Beginner
How strong a building block is at pushing its signal: a stronger one switches a big load faster, but it is bigger and uses more power.
Novice
The variants of one logic function built with wider transistors (in SKY130: inv_1, inv_2, inv_4 and so on). A stronger cell drives more load capacitance with less delay, but takes more area and presents more input capacitance to whatever drives it.
Expert
Wider (or more fingered) devices lower the cell’s effective output resistance, which is the slope of delay against load in its NLDM table. Families share a Liberty cell_footprint so optimizers can swap sizes in place. Upsizing one cell slows its driver, which is why sizing is a path-level problem.
Explained in From devices to a cell library (Transistors).
See also: Standard cell, NLDM (non-linear delay model).