$A \lor B$ is true whenever at least one of the disjuncts is true. Since $T$ is by definition always true, so is $\neg p \lor T$.
Convince yourself by drawing a truth table:
| p | ¬p | T | ¬p ∨ T |
| 1 | 0 | 1 | 1 |
| 0 | 1 | 1 | 1 |
As you can see, $p \lor T$ is true under all assignments of all variables (here: only $p$).
Additionally, as @Wore said in the comment, $p$ stands as a variable for any proposition, including $\neg p$, i.e. the identity will hold for the proposition's negation as well.