It may have more than one meaning, but it is usually a unit. An apostrophe sign can be used in: angles - then it means ' - angular minutes, '' - angular seconds. Example: 75^o20'39'' which stands for: 75 degrees, 20 minutes and 39 seconds. lengths - then it means: ' - feet, '' - inches like: He is 5'4'' tall. The sentence would mean : He is 5 feet and 4 inches tall. (about 165 cm) More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.