Review queues (also known as review tasks) contain posts that possibly need community attention, as determined by the system or other community users. You are shown these posts one at a time so that you can evaluate what, if any, action is needed.
The first queues you gain access to are Late Answers and First Posts. These and all other review queues are accessible from the "review" link that appears at the top of every page next to your user name, though you will only be able to perform reviews in the queues open to users of your reputation level.
The Late Answers queue contains answers which were posted by new users much later than when the question was asked. Late answers tend to be seen by fewer people than answers posted soon after the question was asked, so the review queue helps ensure that these answers meet the same quality standards as all other answers.
The First Posts queue contains the first few posts asked by new users, who may not fully understand the best way to ask and answer questions, or what's on- and off-topic on this site. The purpose of this queue is to give special attention to users who may need to be educated on some aspect of our model and to posts that are more likely to need improvement.
In these queues, you can edit the post, flag it, upvote or downvote it, add a comment, or upvote a previously existing comment. When you take one of these actions, the "I'm Done" button becomes enabled, allowing you to complete the review. If you feel that the post is fine as it is, but you don't want to vote on it, click the "No Action Needed" button. If you're unsure about the post, use the "Skip" button to move on to the next review item without taking action on the current one.
Note: These two queues do not exist on Meta sites.