I don't have the rules with me, but I think it is any particular resource that can only be taken by 1/2 players. ie if player 1 takes wood and player 2 also takes wood, player 3 cannot take wood, but can take any other resource.
This is the correct interpretation. In a 3-player game, only 2 players can go into EACH of the resources. Once the 2nd player places on Wood, Wood is closed for that turn. Once a 2nd player goes on Stone, then Stone is closed to the 3rd player.
For 2-player games, once a player goes into a resource, it is blocked for the other player for that round.
It is meant to force competition and blocking that would otherwise be absent in games with less than 4 players. With 4, you already have a limit that only 7 meeples can go on any one resource.