An antibody is a protein built from four chains — two identical heavy chains and two identical light chains — arranged so that the molecule looks roughly like the letter Y. The two arms of the Y each end in a binding site, and those two sites are identical. That symmetry matters: one antibody molecule can engage two copies of the same target at once, which strengthens how well it sticks.
The binding site is not a rigid socket. It is a small patch of the protein surface, formed by a handful of loops, whose shape and chemical character complement a patch on the target molecule. The target is called an antigen. When the two surfaces meet, weak interactions — hydrogen bonds, electrostatic attraction, hydrophobic contact, and shape matching — accumulate across the interface and hold the two molecules together. Any one of these contacts is weak; the strength comes from having many of them at once.
Two consequences follow from this picture. First, binding is selective rather than absolute: an antibody binds its intended target far more strongly than it binds unrelated molecules, but the distinction is a matter of degree, not a perfect switch. Second, because binding depends on the folded three-dimensional surface, knowing only the sequence of amino acids is not enough to predict whether an antibody will bind — you need to know how that sequence folds.