The sepals are often small and green. Some flowers do not have petals. Then the sepals may be big and colorful. In some flowers, the petals and sepals look the same. They are both called tepals.
All the sepals together are called calyx or 'flower cup'. The number of sepals is different for different flowers. Both petals and sepals surround the reproductive parts inside the flower. The term for sepals plus petals is the perianth.
The main job of the sepals is to keep the flower safe before it opens. At first, the flower bud sits inside the sepals, then the flower opens and sits on the sepals.