From Traderpedia

Definition: A portion of your commission fee that is levied by and paid to the exchange.

Every Exchange charges a fee for every contract traded on their markets - this is usually bundled in with commissions for a retail trader and since retail traders generally pay higher commissions, the fee is not significant.

Professional traders may be elegible for rebates and often pay comparatively much lower commissions for their high volume trading - the exchange fee is usually a significant portion of their trading fee's.