He means that it is an Ab chord with the 4th (a C#) being played. "Sus" means suspended (?) and you suspend or hold that scale degree, in this case its the 4th (C#). If it said Absus2 or Ab2 then you hold the Bb because its the 2nd note in the scale of Ab. Got it!??!
Its important that you learn your chord names. That way if you are in a rehearsal, someone can just say Bbmaj7 (#5) and you will know exactly what they mean rather than them spelling out the entire chord. Im not sure where you can go learn them, but the internet is almost neverending.
Be blessed