Here's what Strong's says:
H5542
selâh
seh'-law
From H5541; suspension (of music), that is, pause: - Selah.
H5541
sâlâh
saw-law'
A primitive root; to hang up, that is, weigh, or (figuratively) contemn: - tread down (under foot), value.
I've always thought of it as just saying "Stop and think about what was just said. Its important.