Flex Pitch is Logic Pro X's answer to the many other options for manipulating audio. It is mainly used to adjust the pitch of a voice. This only with a specific piece in the voice and with a certain pitch height.
Autotune software such as that of Antares and voice correction software such as Medolyne are external plug-ins that you can use for this. Flex Pitch is the function within Logic Pro X that makes this possible without other plug-ins.
Flex Pitch is used a lot to, for example, double voices and adjust them so that it sounds like a choir.
Do you not have a Logic Pro X but do you use FL Studio for example? Then take a look at Medolyne or Newtone.
To enable Flex Pitch in Logic, click on "edit" at the top of the menu and then on "show flexpitch / time" the shortcut for this is cmd + F.
Then click on the track that you want to adjust and you can move the bars further down or up in the "edit" screen. We recommend experimenting with this because you can only pitch the beginning or the end up.
Below is a tutorial in which they elaborate on Flex Pitch within Logic Por X.