As an example, if you are a developer for a mobile app that relates to the restaurant industry, your employer may ask that you don't go and work for a competing app company targeting restaurants. These agreements are not always enforceable and are even illegal in some states.
However, when enforceable, they must be reasonable in length and scope. A non-compete contract that states, for example, that you can never work for another app company anywhere in the United States for the rest of your life would be ridiculous. Again, the accepted length of time on such agreements is usually a year or two.
If you feel uncomfortable signing such a form, or if the verbiage jumps out at you as being unreasonable, contact a local attorney today and have them review the contract. It's always best to be safe than sorry.