Why are contacts in my list grayed out?

If contacts in your list are grayed out like this...

It is because in your List Settings, you choose to use the "List users are allowed to work only with leads/contacts they own" or "List users are allowed to work only with contacts that belong to accounts they own" checkboxes... 

The owner of the list will see everything in the list, but any contacts they don't own will be grayed out. For the other users assigned to the list, they will only see their particular leads/contacts. Only the owner can see everything.