A remake, in video games, is usually a rewrite on a more powerful platform. You get better resolution, better music and event better gameplay.

A demake is a rewrite on a less powerful platform.

For someone unfamiliar, it's also worth noting this term is used pretty loosely. (I'm guilty of this too!) E.g. Before Rivals of Aether, the same dev made a Smash clone in the GameBoy artstyle and controlset. It's understood as a 'demake' but it's a PC game and is designed for platforms way more powerful than an N64.