Player ratings are a number indicating the ability of a player.

While form is a description of a players past performances, often over a short period of time, a player rating is a indication of their overall ability. 

Ratings are reviewed on a monthly basis and all changes are shared with the community.

If you own a player and their rating changes, their salary will remain at level it was when you signed them. The cost to renew their contract (in Transfer Coins) will always be based on their current rating.