This is the 11th most frequent Russian word.
до
"До" means "until," "before," or "to."
In this sentence, "до" is used to indicate a destination or a limit in terms of movement towards a particular point.
Мы шли до реки.
We walked to the river.
Here, "до" is used to denote the time frame leading up to an event, specifically it means 'before'.
До прихода гостей осталось немного времени.
There is little time left before the guests arrive.
In this example, "до" indicates a time limit, specifying a deadline by which something should be completed.