also: drive block · base · man block
The man in front of you, no help, move him back. The block everything else in football is defined against, and the one with the most names.
2 meanings, all below. Jump to the one you heard.
A lineman blocks the defender aligned over him, on his own, and tries to move him backward or at least hold him where he is. No angle, no double team, no pull. It is the simplest assignment in football and the hardest to execute, because it is the only one that asks a lineman to win without any structural advantage.
That is why it is the reference point. Every other block in this cluster exists because somebody decided that asking a lineman to win straight up was a bad bet, and built an angle, a second body or a different direction into the assignment instead.
drive block, man blockdown block, reach blockIn a blocking scheme written as rules rather than as assignments, "base" is often a conditional instead of a technique: block the man over you if one is there, and if not, do the next thing on the list. A lineman with nobody over him is uncovered, and his rule sends him somewhere else entirely.
covered and uncoveredDrive block and base block are used interchangeably by most coaches, and where a staff distinguishes them the usual split is that drive names the aggression and base names the assignment. Establishing which your staff means is a first-week question, not a rulebook one.