what is an anode?

Any time you have two different metals that are physically or electrically connected and immersed in seawater, they become a battery. Some amount of current flows between the two metals. The electrons that make up that current are supplied by one of the metals giving up bits of itself — in the form of metal ions — to the seawater. This is called galvanic corrosion and, left unchecked, it quickly destroys underwater metals.
Common casualties of galvanic corrosion are trim tabs. The way we counteract galvanic corrosion is to add a third metal into the circuit, one that is quicker than the other two to give up its electrons. This piece of metal is called a sacrificial anode, and most often it is zinc or aluminium. In fact, most boaters refer to sacrificial anodes simply as zincs.
It would be hard to overstate the importance of maintaining the anodes on your trim tabs. When an anode is missing or largely wasted away, the metal component it was installed to protect begins to dissolve.
When purchasing your trim tabs, remember to buy anodes.