Ferts is short for Fertilizers. In low light tanks your plants can usually get enough nutrients from the tapwater and fish, but in higher light tanks the plant grow so fast that they start running out of things. To keep things in balance and avoid algae, you need to add fertilizers to keep the plants from running out of what they need.
There's a link in the Read this First Sticky that has lots of information on Fertilization.
Also, with that much light you'll probably need to consider CO2