The Tinder "Queue" is the list of potential matches which are queued ready for you to swipe left or right to indicate whether or not you like them.
Who sits in the queue is indicated by the parameters you set for the app. For example, you can indicate if you want to see only guys, only girls or both. You can also set an age range- so if you only want to see potential matches above 25, you can specify this here, along with a distance range in miles or kilometers.
When you have swiped through your queue and there are no more potential matches for you to judge, the app will let you know, and then when you close Tinder they will start to build up again when new people join or enter the distance range you have set.