That is an easy one!
Many neighborhoods have utility easements. This is a section of your lot that the utility companies have the right to use to access, repair, or replace their lines. It could be in the front, on the side, or at the back of your lot. It belongs to you, and you are responsible for the maintenance. Although it is yours, if you build something like a shed or put up a fence in that area, the utility company or the City can take them down to gain the access they need. They will not replace these structures after they are finished.
When you buy a home, it is a good idea to find out where these easements lie, so that you know where you can safely put your fence or shed in the future. If that information is not readily available, you can request a Survey or Improvement Location Certificate (ILC) which will show where these easements appear.