The little apostrophe can make a big difference in meaning. The following sign at a neighborhood swimming pool, for instance, probably doesn’t say quite what the writer intended:

Adding a single apostrophe would offer a more neighborly statement: Please deposit your garbage (and your guests’) in the trash receptacles before leaving the pool area asks residents to remove the guests’ garbage, not the guests themselves.

Apostrophes are used for two main purposes: to show the possessive case and to show where a letter has been omitted from a contraction. Apostrophe placement can be confusing, especially today when smart devices sometimes put them in for you, and they are sometimes left out in informal writing. But they are still important in writing, so it’s best to learn how to use them accurately.