A margin, with respect to trading contracts for difference (CFDs), is the amount of financial security (i.e. money) that the CFD broker requires you to have with their accounts in order to enter in CFD trades. The margin works by calculating a percentage of the underlying reference security which you must keep in your trading account. Also note that since the markets are fluid and are in constant motion – fluctuating up and down, the CFD trader’s margin obligation also fluctuates as their CFD’s margin requirements are a percentage of their price. Another reason your CFD margins would fluctuate would be in the case where you are trading CFDs denominated in another currency other than your original base currency and hence your margin requirements would also fluctuate with the changing forex rates between your base currency and the host currency.