Life, death, and rebirth.
Ouroboros (or uroboros) — an ancient symbol depicting a reptile eating its own tail. It symbolizes fertility in some religions. The tail is a phallic symbol (an erect penis). The mouth is a yonic (vagina) or womb-like symbol.
In real life, it happens.
Although not common, I have seen snakes eating themselves. Scientifically, autocannibalism (self-cannibalism) in snakes is not normal. Well so, snakes get disoriented when overheated, and their metabolism goes into overdrive. It makes the snakes feel so hungry to the point that they will try to eat the first moving thing they see, and that’s usually their own tail. When the feeding mechanism starts, snakes will have their focus fully on consuming the prey. The feeling of pain will be overridden. Many snakes will keep going even if they’re bit, mauled, and even beheaded.
You can see why ancient Egyptians chose snakes to symbolize the eternal cycle of life, death, and rebirth.