Bilateral symmetry defines a characteristic people see every day, but never think about. Evolution almost demands that advanced, free-moving animals express this type of symmetry.
Merriam-Webster gives this definition: symmetry in which similar anatomical parts are arranged on opposite sides of a median axis so that one and only one plane can divide the individual into essentially identical halves. But a clinical definition doesn't really give people a sense of what they're looking at.
Think of your own spine, how it's in the center of left and right. With that as a median axis, you could divide a person in half and clearly see that the left and right side are nearly identical. Each has an arm, a leg, toes, fingers, an eye, an ear, and so on. Even the organs are basically mirrored, though sometimes it's just a matter of balancing; one side gets the liver, the other side gets the gall bladder.
If you made a similar cut, only defining front and back, the mirroring is not so apparent. Muscle groups are different; one side gets more fingers, and the other has no toes; organs are no longer balanced and grouped. Over all, nothing seems complete if you separate along a different axis.
Look at animals, and see how their body plans are balanced along the spine. Two left legs, two right legs, one ear, one eye, and the same balancing you find in humans. If you were to split a dog top to bottom or front to back, the mirroring is gone. Even if you end up with two legs in each half, the all important mirror is gone.
The functionality of the animal is also entirely compromised. If the same sample dog loses one leg, he can continue to walk and run and play, because there is balance. If you gave him just his two front legs, he's not going anywhere. Dividing along any line but the spine ruins the creature, and disrupts all functions. Birds couldn't fly, fish couldn't swim, snails couldn't slide along. The very functionality of everyday life is based on mirrored sides.
Bilateral symmetry is the foundation of the animal kingdom on Earth. It binds people to this world the same as it does lions, and tiger sharks, and water bears. From the microscopic to the macroscopic, animals are balanced on their median axis, two halves coming together for a perfect whole.