Personal Contacts is a personal phone directory on your phone. It's a great way to keep a list of people that you call often, but don't call so often that you need to set them as a speed dial.
Press the Contacts button (shown below), and press the Add New soft key. Enter the name and number and press Save.
For a faster way to add an entry, simply call the person (or wait for a call from them). Then, after the call, press the Call History button (shown below), and use the silver navigation button to select either Outgoing or Received, find their name and number. Once they're highlighted press the Add Contact soft key.