A bond length is simply the equilibrium distance between the centers of two bonded atoms. It is commonly reported in angstroms (Å) or picometers (pm).
Bond length is fundamentally dependent upon the covalent radii of the bonded atoms and the bond order. Higher order bonds tend to be shorter bonds. Covalent radii decrease as you go up and to the right on the periodic table. Smaller atoms make shorter bonds.
Smaller atoms also tend to have higher electronegativities, so more electronegative atoms tend to form shorter bonds.
If you think of hybridization in terms of the percent s-character, higher percent s-character orbitals make shorter bonds. So, sp2 orbitals are 33% s-character, while sp3 orbitals are 25% s-character. Thus, sp2 orbitals tend to make shorter bonds.