In an acid-base titration, 0.1MHCl solution was added to the NaOH...

For titration of strong base with strong acid $pH$ changes sharply near equivalence point and at equivalent point $pH$ sharply decreases because strong base & strong acid dissociate completely.
$\overset{SA}{HCl} + \overset{SB}{NaOH} \to NaCl + {H}_{2}O\left(l\right)$
at equivalent point $pH = 7$ and at endpoint solution will be neutral.