Pinch gripping involves squeezing the extended fingers towards the thumb without flexing individual digits, such as you'd experience holding a textbook.
Most strength athletes use plate pinches (holding two 25 or 45 pound plates together), but there are alternatives. For example, using two dumbbells allows you to have access to a greater range of weights. That makes it much easier to progress in smaller increments.
If you've been around weight training for any length of time, you've probably read or tried an arm specialization routine. There are a bunch of these around with different cool names for what amounts to the same old shit: do a bunch of arm exercises. Yeah, the exercises change and the sets and reps change, but it still amounts to just doing more arm work.