The "proportional Part" defines a relation that 'how two different variable in an equation are connected' .
i.e. if x and y are two variables in an equation , then proportional means if we increase the value of x then the value of y will be affected by this increment in the value of x.
e.g. if we take a fixed amount of an ideal gas and consider its pressure to be constant. Then by Charles Law
if we start heating the gas i.e. if we increase the temperature of the gas, its volume starts increasing(i.e. the gas will start expanding).
or if we decrease the temperature of the gas, its volume will decrease.
So, here the volume V is proportional to the temperature T .