YouTube like many other websites and services on the Internet uses a rating system that enables users to vote for or against videos on the site. While I can't say for sure if and how these information are used by the site, it is likely that it is one signal that is being used to calculate a video's popularity and exposure on the site.
While I'm not a fan of user based rating systems they sometimes can be useful nevertheless. The main reason why I dislike them is that they can be manipulated easily in many cases to artificially raise an items popularity on a site. It is also possible to abuse the system by down voting items.
Generally speaking, ratings can help you in your decision finding process but only if you do not rely on them solely for that. On YouTube, they may help you decide whether to load a video or not on the site. You may for instance make the decision to skip a video that has received a lot of down votes. While it may make sense to load it anyway if the title, description and thumbnail look okay, you may prefer to load a different video that looks similarly suitable.
Ratings on YouTube are displayed on each individual video page which does not really help you at all make a decision before you open the video. The Google Chrome extensions YouTube Ratings Preview (also available for Firefox) comes to the rescue by adding a visual rating indicator to all pages on YouTube where video thumbnails are displayed on.
The extension adds a ratings bar beneath each video thumbnail on YouTube that is displaying green and red rating indicators. Green indicates up votes for a video and red down votes so that you can assess each video's ratings on first glance. Note that this does not tell you anything at all about the number of votes, which may be useful for the decision making process. A video that received one up vote for instance displays a bar that is purely green. You can use the views it has received as a basic indicator of the number of ratings it should have received.
If you hang out on YouTube a lot and like to use video ratings as part of your decision making process whether to watch a particular video or not, then this will certainly assist you with that.