An all-seater football ground or stadium is one where every person watching has a seat — nobody stands in a cheap standing area. You will hear this most in British football. For a long time many grounds had terraces where fans stood packed together; after serious crowd trouble, the big English stadiums were changed so that everyone sits. The word is almost always about the ground, not about a person: an all-seater stadium, an all-seater ground. It is a sports-news and planning word, a little official. You would not use it of a cinema, which already had seats. Though actually, fans still argue about it: sitting is safer, but some people miss the noise and feeling of standing together.