Quantizing aligns the timing of MIDI notes or audio material to a grid of beats or note values. It tidies up loose timing and makes rhythms line up. On the other hand, aligning everything perfectly can sound mechanical, so adjust the amount to suit your purpose.
What gets aligned
Usually the start positions of notes are moved toward quarter, eighth, or sixteenth notes. Some DAWs can also target note ends, audio attacks, and incoming input while recording. Choose the division to align to according to the rhythm of the song.
Do not apply it too strongly
At 100% the notes move exactly onto the chosen grid. If there is an Amount or Strength control, start from a small value to keep the nuance of the performance. Also check whether the edit is non-destructive so that you can restore the original timing.
Keeping the groove
Slight pushes and pulls in the drums and bass can be what gives a song its feel. Instead of treating every part with the same strength, tighten the main rhythm and fix only the spots that need it elsewhere. It is important to judge with your ears rather than by what the screen looks like.
Velocity is a separate matter
Even with the timing tidied up, notes that are all the same strength sound unnatural. Check the velocities as well and keep the accents. Quantize concerns rhythmic position, while velocity is separate information about how hard a note sounds.
The order to work in
Listen to the performance first and fix only the timing that bothers you. For the basics of MIDI editing, see the article on DAWs.
