it means to make the equaiton equal to 1y with only 1y on one side and no other y's anywhere else, and have x on the other side
example
express y in terms of x
-2x+5y=20
isolate y
add 2x
5y=2x+20
divide both sides by 5
y=2/5x+4
that is the equaiton that does y in terms of x