Absus2sus4 is Ab, Bb, Db and Eb — Absus4 with the second added underneath the fourth.
It sounds ringing and deliberately ambiguous. Read another way it is a stack of fourths, which is why it works as a pad rather than as a chord that resolves.
Texture more than function — still no third.

| Ab | Bb | Db | Eb |
|---|---|---|---|
| 1 | 2 | 4 | 5 |
Also written Absus2/4 or Ab suspended 2nd and 4th. The same chord either way; which spelling you meet depends on who wrote the chart.
The easiest grip here is 4413xx — no barre. None of the shapes for Absus2sus4 uses an open string, so every one of them is movable; the lowest sits at the 1st fret. 4 of the 6 need a barre. Together they cover the neck from the 1st fret to the 13th.
All 6 common shapes as fret numbers, lowest string first. Hover or tap one to see it drawn.
In which keys. Every note of Absus2sus4 belongs to C# major (the V), Eb minor (the iv), F minor (the ♭III), F# major (the ii), Ab major (the I), Ab minor (the i), Bb minor (the ♭VII) and B major (the vi).
Every note of Absus2sus4 is in Ab major, Ab mixolydian, Ab dorian, Ab natural minor and Ab harmonic minor.
Change one note of Absus2sus4 and you have Absus2 (drop the fourth), Absus4 (drop the second), Abadd9 (the fourth becomes the major third), Abmadd9 (the fourth becomes the minor third), Ab7sus4 (the second becomes the flat seventh) or Ab11 (add the flat seventh).
Absus2sus4 is a suspended chord — root and fifth, with a second or fourth where the third would be — or nothing at all. Suspended and power chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.