A suspended chord, often abbreviated as "sus chord", is a triad chord where the third, either a major or minor one, is omitted.
For a suspended fourth chord, it is substituted with a perfect fourth.
Because of the missing third, the chord has an open sound but also tension because of the present dissonance between the fourth and fifth.
Below, you can find a list of all suspended fourth chords available on ChordAtlas. Clicking on the respective entry will lead to the detail page of the selected chord: