In VSEPR theory, we are interested in the geometry around a particular atom typically referred to as the central atom. The goal is to determine the geometry of the atoms that are bonded to this central atom. To determine this, we must first determine the electronic geometry. The electronic geometry is an idea of where the regions of electron density are surrounding the central atom. A region of electron density can either be bonding or non-bonding. Electrons in a covalent bond between two atoms are counted as one region regardless of whether it is a single, double, or triple bond (or anything in between). Non-bonding regions are counted by counting the number of lone pairs on the central atom.
The electronic geometry is determined by the geometry that minimizes the repulsions between these regions (moves them as far apart from each other as possible). This geometry depends on the number of regions. If there are two regions, the farthest apart they can get from each other is to be on opposite sides of the central atom. As the two regions and the central atom would now all be in a line, we call this electronic geometry linear. Similarly, the farthest apart we can get three regions is if each is 120° apart from each other forming a triangular shape in the same plane. We call this geometry trigonal planar. Four regions leads to a tetrahedral geometry, five a trigonal bipyramid, and six an octahedral geometry.
We can then use the electron geometry to determine the molecular geometry (where the nuclei are).