An apostrophe sign can be used in:
angles - then it means ' - angular minutes, '' - angular seconds. Example: $$ 75^0 20'39"$$ which stands for: 75 degrees, 20 minutes and 39 seconds.
lengths - then it means: ' - feet, '' - inches like: He is 5'4'' tall. The would mean : He is 5 feet and 4 inches tall. (about 165 cm)