Simplifying a Stochastic Integral to a Summation

Question

I am writing my undergraduate dissertation on Self-Exciting Processes where the conditional intensity function is expressed as: $$\lambda_t | \mathcal{H}_t = \lambda_0 + \int_0^t \phi(t-s)dN_s = \lambda_0 + \sum_{T_i \leq t}^{} \phi(t - T_i),$$ for function $\phi: {R_+} \to {R_+}$. My question is how did we go from the stochastic integral with respect to the point process (N_s) to the summation? The books I have checked do not explain this transition. Thank you :)