Your mother was only partly right. The apostrophe is also used to indicate possession, in simplest of terms it goes before the s for a single possession, after the s for plural possessions. There are of course exceptions (!), e.g. if the word already ends in an s, or if it is different in its plural form. And, as shown in the previous sentence, “its” indicating possession does not have an apostrophe, where as “it’s” meaning “it is” does. English is a wonderful language!