mmmmm i think the attached photo will explain it
in the first situation function = u(t)
it is only a step function which will equal 1 for all t >0 and zero otherwise
in the other figure
function = a2 * u(t) where a2=5
then the function will equal 5 for all t >0 and zero otherwise
i hope u got it ......sorry for the hand drawing figures
The first equation represents a ramp starting in time from -infinity going to +infinity. The second equation is the same ramp (if a1=a2) but time limited from 0 to +infinity; in this case h(t)=0 if t<=0.
Usually real signals are starting from 0; think, for example, at a circuit that is switched on at a certain time instant (this is t=0).