If you stand in for someone, you take their place or do their job, because they are ill or away. I had to stand in for her on Tuesday when she didn't show up. the acting president, who's standing in while she's out of the country. see also stand-in
one who comes to change the place of a permanent worker that has left temporarily
be a substitute; "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream--we are on a strict diet"