Chirality is a geometric property of certain molecules. A molecule is said to be chiral when its mirror image is not superimposable to the molecule itself. The classical source of chirality in a (organic) compound is the presence of a carbon atom with four different substituents.
The origin of chirality, together with the origin of life is one of the most relevant questions in, not only chemistry, but science in general, so it is not possible to answer in a straightforward manner. The main theory backing it up is based on homochirality, which could have emerged over three steps: mirror-symmetry breaking, chiral amplification and chiral transmission. This is far beyond basic chemistry concepts.