How does the hydrogen ion (H+) [or the hydronium ion (H3O+)] and the hydroxide i

It is impossible to help you without knowing the initial reaction that this question is referencing.

let the acid be AH

and the base be BOH

so AH -----> A- + H+.......................................(1)

and BOH -----> B+ + OH-.......................................(2)

(1) + (2)

=

AH + BOH ------> A- + B+ + H+ + OH-

AH + BOH ------> AB + H2O



consider an acid thich is to be titrated with a base
when there is pure acid the concentration of H+ is high as compared to OH- so the indicator combinse with small amount of H+ to give the colour
when we add base to it the H+ ion concentration starts decreasing and OH- starts increasing nut thw colour remains same
there is point when H+ concentration = OH- concentration
after this point OH- concentration is more and the indicator is no longer combined with H+ and hence colour changes