Your trade fee depends on the currency pair you are trading, and it is reduced according to the USD value of the total volume you've traded over the past 30 days.
New trades are added to your 30 day volume immediately, but trades older than 30 days are only removed every few days. This means that sometimes you might temporarily get a lower fee than you should, but you won't get a higher fee than you should.
"Current Fee" and "Next Fee" (in the upper right corner of the "Trade" tab on your account screen) indicate your fee status relative to your trading volume. "Current Fee" indicates the fees you are currently being charged, and "Next Fee" shows the level your trading fee will drop to if you reach the next trading volume tier. The progress bar in between these two fees indicates how much more volume is needed to drop to the next fee level.
In the example above you are currently paying 0.16% for maker orders and 0.26% for taker orders and if you traded $50,000 more within the next 30 day rolling window the fee would lower to 0.14% maker and 0.24% taker.
Important: USDT (Tether) trades do not count towards your 30 day volume.