NEED HELP! how do i get to eternal mode and what does it do?

I am guessing you are talking about Me and My Katamari?

After each stage, the King will tell you your score, your size, the time you took, and then a second score from 0 to 100. For any stage where you get that second score up to at least 80, you earn Eternal. After that talk to the animal again, and the animal will offer you eternal mode.

Eternal mode lets you play without a time limit.

Hope this helps.