As the name implies, bond angles are the angles between the bonds of the atoms bonded to the central atom.
Bond angles differ between each different VSEPR shape. All molecules with a linear electron pair geometry have a bond angle of 180°. Molecules with a trigonal planar shape have bond angles of 120°. Molecules with atoms bonded in a tetrahedral shape have bond angles of 109.5°. If a molecule have atoms bonded in the trigonal bipyramidal shape, the bond angles between the three atoms in the center plane have bond angles of 120°, while the bond angles between the an atom in the center plane and an atom on the vertical axis are 90°.(see below image to the right) Molecules with atoms bonded in the shape of an octahedral have bond angles of 90° in the central plane and bond angles of 90° as well between atoms in the central plane and the vertical axis. (see below image to the left)
Both CH4 and NH3 have their electron pairs arranged in a tetrahedral whether the electron pairs are used in bonding or are lone pairs. One would expect that both molecules would have the same bond angles, 109.5° between each bond. While the angles between each bond in CH4 are 109.5° between each bond as suspected, the bond angles in NH3 are about 107° instead.
The image on the left shows that bond angles for molecules with the same electron pair geometry will decrease as the amount of lone electron pairs increases. Lone electron pairs need more space which will push bonds more together decreasing the angle between them.
The reasoning for the greater space needed for lone electron pairs is due to their position around the atom's nucleus. When a pair of electrons are used in bonding, the electrons are located in a space between the two bonded atoms. Lone electron pairs, however, are located around the central atom and are spread out around it. This means that lone electron pairs will take up more space arouund the central atom than electrons used in bonding thus accounting for the decrease in bond angles when lone pairs are present.