When Flex Pitch is turned on in the Audio Track Editor, you can quantize, or automatically correct, the timing of regions on an audio track. This is especially useful when regions on the track contain the right notes but are not perfectly in time with the project tempo.
When you quantize the timing, selected regions on the selected track are adjusted to the selected note value. You can quantize the timing of regions with drums, single-note instruments, and chordal or polyphonic instruments. If you aren’t sure which note value to use as the basis for timing quantization, a good starting point is the note value on which your beats fall; in other words, if the beats in your project normally occur in 1/16th note intervals, set your note value to 1/16th notes. If the resulting quantization doesn’t sound or feel right to you, try again using the next higher or lower note value, until you are satisfied with the results.
Click the Show/Hide Flex button 
in the Audio Track Editor menu bar.
Choose Flex Pitch from the Flex pop-up menu in the Audio Track Editor menu bar.
Select the regions you want to quantize.
Choose the note value to use as the basis for timing quantization from the Time Quantize pop-up menu.
Drag the Strength slider to the left to decrease the strength of quantization.
For more information about Flex editing, see Flex Time and Pitch overview.