Brushing your horses increases their happiness levels. Brushing other players horses also increases their horses' happiness and yields you cash for doing so.
There are different brushes available e.g. Star Brush and Sonic Brush which need to be unlocked by completing the Groomer Achievements.
1. Visit your stable by clicking on the Stable button at the top of the game (or select the Stables from the Town map).
2. Click on a horse in your stable to open up the horse profile window
3. Select the 'Brush" icon from the "Actions" menu.
You can also brush your friends horses by selecting the "Friends" stable from the Town Map and following the same procedure as above.
The improved brushes (Star and Sonic) appear in the Shop every now and then. Generally, the more expensive the brush, the greater the effect on the horses happiness and the more cash it will yield when you brush other players horses.
The Sonic brush can yield up to $2,000 per brushing!