What are True Leaves?
When you grow vegetables and herbs from seed, they first emerge and sprout a set of two leaves. These are not the true leaves, these are known as cotyledons. The cotyledons are what actually started as the seed itself and it provides nutrition to the seedling as it grows. Photosynthesis hasn't kicked in yet, so it's not getting it's energy from the light yet. Once you see leaves that start to look like the plant will eventually look when it matures, THAT's when it's starting to photosynthesize.
I'm so impressed by this process I posted about it on Instagram. You can see in this photo of basil that there are two sets of leaves on the seedling that look like basil is supposed to look, and two that don't. These represent the cotyledons and the true leaves together.