Do you mean a "scale" - in which there are labels at each end such as: 1:5, 1:10, 1/2"=1", etc.? Possibly triangular in shape or simply flat, but with fewer scales on them?
Something like this?
If so, what you do is look for the scale which is given on the drawing. If it were 1:75 then you would lay the scale down on the drawing on the part that you wanted to measure. The scale works like a normal ruler except that the spacing of the lines have been adjusted for the particular scale noted. That way, if the drawing measures "2" then the item you are measuring is 2 metres long.
Scales can be metric or imperial. If they are metric, they have nice round scales like 1:5, 1:10, 1:20, 1:50, 1:100, etc. (1:75 is actually a bit of an oddball). If it is imperial, it willl read with "inch:feet" notation, like: 1/2" = 1', or 3/8" = 1'.