For all practical purposes, these days, it simply means “focus on this” or “now call your attention to this”. During the Mass it usually means “stand up now”.
Wikipedia had this good entry:
Oremus (Latin for let us pray) is the invitation to pray, said before short prayers in the Roman Catholic Mass
…
“Oremus” is said (or sung) in the Roman Rite before all separate collects in the Mass, Office, or on other occasions (but several collects may be joined with one Oremus). It is also used before the Post-Communion, the offertory, and before the introduction to the Pater noster and other short prayers (e.g., “Aufer a nobis”) in the form of collects.
It appears that the Oremus did not originally apply to the prayer that now follows it. It is thought that it was once an invitation to private prayer, very likely with further direction as to the object, as is now the case in the liturgy for Good Friday (“Oremus pro ecclesia sancta Dei”). The deacon then said: “Flectamus genua” (“let us kneel”), and all knelt in silent prayer. After a time the people were told to stand up (“Levate”), and finally the celebrant collected all the petitions in one short sentence said aloud (hence, the collect). Eventually this was simplified into the word alone preceding the prayer.
