Websites can request several configuration settings from your browser in order to help them printing the best page format for you. 

But by requesting your browser's specific abilities (here canvas is used for graphical rendering), websites could fingerprint your browser (if you have a unique configuration) across multiple sites. 
This is related to the Panopticlick (https://panopticlick.eff.org) method the EFF exposed, using other information sent by your browser to uniquely identify you, even without access to your IP adress. 
The vast array of information sent makes it easy to end up with a very unique, thus identifiable, signature. 

Tails (https://tails.boum.org/), for example, can help you to make yourself look more like other users. And disable JavaScript which would allow a lot more info to be sent. Do the Panopticlick test with and without JavaScript (erase the cookie between tests). 
If you must use JavaScript or plug-ins such as Flash, you must use a more secure set up, such as Whonix. 

Also, you have to bear in mind that against a very large and capable adversarie that listen to all Internet traffic, it can easily follow that type of trace if it exists; it's not only the individual websites requesting this information (which 99% is legitimate, just needed to improve rendering) that you should be wary of but the global big brother that is always looking over your shoulder.