Boyle's Law is a relationship between pressure and volume at a constant temperature.
#P_1V_1 = P_2V_2#
In this relationship, pressure and volume have an inverse relationship when temperature is held constant.
If there is a decrease in the volume there is less space for molecules to move and therefore they collide more often, increasing the pressure. If there is an increase in the volume the molecules have more space to move, collisions happen less often and the pressure is decreased.
vV ^P ^V vP the relationship is inverse.
However if temperature is involved the relationship between the three values is the combined gas law.