What is a Steep or mellow kick?

Mellow

Most decks have a mellow kick. A mellow kicktail is more relaxed for street tech skater because it is more responsive and you will change your foot position more often. If you're just starting out, we recommend trying a mellow kicktail

Steep

Steep kicktails give you more pop, but it also takes more effort to pop so you can get exhausted faster. With a steep kick you do have a better feel for where your foot is on the deck which can be useful in a pool, mini, vert.

It is also a matter of personal preference, and you cannot make a wrong choice here.