"A U.S. citizen who's a resident of a foreign country or countries for an uninterrupted period of time during the tax year."What does it mean by "resident?"

Resident in this case simply means living in a foreign country (or countries).  It doesn't require any sort of residency "status" in the foreign country such as permanent resident or the like. 

Uninterrupted period here means continuous time away from the United States, except for short visits for vacation or similar (less than 30 days total, typically).  However, the time isn't the only thing that helps define the uninterrupted period of "residence", either.  Other facts and circumstances can come into play. 

For example, if you have moved to another country and you returned to the US for a week (or evan a few weeks) to visit family, celebrate holidays, or any other reason you would still be considered to be living abroad for an "uninterrupted period".  

However, if you move out of your home abroad completely, and back to the US even for a short time before returning abroad (to the same or a different country) that wouldn't be "uninterrupted" residence abroad.