Basically, you and one other person play on a board of 25 letter tiles. All tiles are worth one point. Words are made by tapping the letters. Words cannot be prefixes of already played words (e.g. if I play “horseradish”, you can’t then play “horse” — but you could play “radish”).
When you play a word, the tiles become your color. If you use a tile and surround all its sides, that tile becomes protected, mean I can use it, but don’t get points from it. The game is over when all tiles have been used, and whomever has the most tiles in their color wins.
Here’s an example of how a Letterpress game plays out.