Here is an example of two types of defensive pass coverage.
Defenses use manys ways to cover the receivers. This is a zone
coverage.
A "Cover 2" defense breaks the deep secondary into
halves. The safeties (FS & SS) each cover a side. The corners
(RC & LC) cover short zones. The linebackers (LB) cover the
curl routes underneath. The "seams" are the areas
between the zones.
A "Cover 3" defense plays three men deep: the two
cornerbacks and the free safety (RC, FS, & LC). The
responsibilities for the short zones change, with the strong side
safety (SS) taking the flat on that side. In this scheme, the
weak-side linebacker is responsible for two zones: the flat and
the curl routes. Versus both of these coverages, the Quarterback
looks to hit the tightend in the seam behind the linebackers and
in front of the safety.