Germination happens when a baby plant is growing. The plant is between the cotyledons. This is a seed. The seed is underground and is collecting nutrients.
When a seed starts to grow, we say it germinates.

The cotyledons store food for the baby plant inside the seed. When the seed starts to germinate, the first thing to come out is the main root. The skin starts to split and the tiny shoot straightens, carrying the cotyledon[s] with it. The main root gets bigger. Side roots appear and so do leaves. To grow, the seed's growing conditions usually have to be damp, warm, and dark, like springtime soil. A dry seed will stay dormant unless it soaks in some water. Then it will start to germinate.
 
 
 

First the young root appears.

The seed coat splits and   the shoot grows side roots, and the first leaves appear.

The main root gets bigger and straighter, pulling the cotyledons with it.