A collar is added to market orders in order to increase the likelihood that your order gets fulfilled. All market orders are placed at the current market price with a collar percent. This collar percent (0%, 1%, or 2%) means that you may be willing to take an opposing order priced within that collar percent difference of your submitted order.
If you're less price sensitive, you can set your collar percent to a higher amount (i.e. 2%) in your User Preferences (Edit Profile - Preferences) and more of your orders will match.
If you're more price sensitive, you can either place limit orders to know you'll get the desired price or you can set your market order collar preference to 0%.
Here's an illustrative example of a collared market order with a 2% collar:
1. Jane places a market order at +105 (or 48.78% probability) for Cowboys to win.
2. The market has an open market order (with 2% collar as well) available for the Cowboys to lose at +100 (50%).
Since the market order is within 2% of the collar, her market order will fulfill. Further noting, she'll get a price at +102.5 (49.38%) allowing her to match with the other user with no fee.