to exit; to go out
The first steps of dancing a tango, or a tango pattern, derived from "Salimos a bailar?" {Shall we (go out to the dance floor and) dance?}.
Start of the paso basico
Nowadays, in Tango it's also used to mean the exit from, or start of, a step, a figure, a pattern etc.
(from tangocanberra.asn.au (http://www.tangocanberra.asn.au/words.htm#Salida))