Take ethylene $H_2C=CH_2$; around each carbon atom there are 4 . Two of these electrons combine with 2 hydrogen atoms (each with 1 electron) to form $2xxC-H$ bonds.
Each carbon now has 2 electrons to play with. 2 of the electrons overlap to form a $C-C$ $sigma$ bond between the carbon nuclei. The remaining 2 electrons overlap above and below the $C-C$ vector to form a $pi$ bond. In the double bond, there is thus extra electron so that the internuclear repulsion between the the carbon atoms is negated and a closer $C-C$ separation can be achieved. $C-C$ typically have $1.54xx10^-10*m$ bond lengths; whereas $C=C$ typically have $1.35xx10^-10*m$ bond lengths.
Where a second $pi$ bond may be formed, such as in acetylene, a $C-=C$ bond length of $1.21xx10^-10*m$ may be achieved.
