Laplace Transform Unit Step Function

Question

I was wondering what is meant by a unit step function in Laplace transform diff equations. Eg : $f''(t) + bf'(t) + cf(t) = u(t)?$ I know that $L[f''(t)]$ is $s^2F(s)-sf(0)-f'(0)$ and so on, but what to do with $u(t)?$