Pass protection is a deep and varied subject, but at least a little can be said to understand the very high-level basics of how the pass rush/pass protection chess match plays out on a given play. Essentially, there are two types of protection schemes: (1) area or zone schemes, and (2) man schemes. Some protections blend these two approaches, either explicitly or implicitly.
(1) Area Schemes: An area scheme is where a group of blockers set up in a given area and then sort and pick-up whatever “trash” comes through. For example, if the center, guard, and tackle are responsible for one side of the protection, and the defense crosses and twists a couple defensive linemen and a linebacker, the blockers will take the one that enters their area. This is probably the soundest “protect-first” approach, and good teamwork will allow the line to deal with defensive creativity with a simple sound approach.
Problems with area schemes arise when you introduce runningbacks, tight ends, or H-backs into the equation. The problem is twofold:
(a) An area scheme could leave you with a terrible match-up, such as a runningback on a defensive end (or Lawrence Taylor).
(b) An area-assigned protector who is also a skill player (like a tight end, H-back, or runningback) has a difficult time releasing into the route if the defense does not blitz. So any of those skill players who you have assigned to an area scheme likely will not get out into the route, and you might only have three receivers trying to get open against seven pass defenders. For example, see the diagram below, where the tight-end and runningback (both skill players and potential receivers) end up in the pass route while the center and right-tackle end up blocking no one at all.
More specifically, the guard, tackle, and Y (TE) are playing an area scheme, making them responsible for the defensive tackle, the defensive end, and the stronside linebacker (Sam or “S”). Although we could handle a stunt or twist, with the middle and strongside linebackers dropping into coverage the tight-end and potentially the runningback have to protect, while interior linemen block no one. Indeed, the tight-end ends up blocking the defensive end, a potential mismatch. There are ways around this problem, but it is a definitely concern.
The most common “area” protection is slide or “gap” protection, where the line all slides to a gap. More on this in a moment.