Ant hills are a sign that you have ants (obviously), and really, many of the ants you see are a normal part of your local ecology. What kind of ants live in the hill? Some ants are completely harmless, and go about their business without ever impacting human beings. Other ants may be aggressive and potentially become pests. Many species lead fascinating lives that involve other insects (cornfield ants tend corn root aphids -- see this link from North Carolina State University), and some ants interact with other organisms (for example, many ant species cultivate fungi underground to feed their colonies. This link to the Ant Colony may be a good place to ask questions about ants.
