Early Bird terms allow you to offer slightly better terms to those who invest first in your round. It's a reward for those who decide to back you before everyone else.
The lead investor should help you to set the Early Bird terms since they're typically one of the first to invest. Generally, Early Bird terms are a 10% - 25% discount off of the normal terms.
For example, if you're raising €250K on a SAFE with a €10M cap, you may decide to offer the first $50K of investors (the "early bird investors") an $8M cap.
Early Bird terms are allocated on a first come, first serve basis as investors confirm their investments in the round.