Wrex is Tali's Uncle

Описание: Wrex is referring to reload cancelling, which stops the long reloading animation for low capacity guns such as the Claymore or Widow. The clip gets refilled before the animation ends, so reload cancelling allows you to skip the end of the animation and fire again sooner.

Personally I don't use reload cancelling, but this is how it was explained to me.
1) Fire until the clip is empty.
2) Watch the bullet count go from 0 to full.
3) Immediately use a power. This will cancel the rest of the reload animation. (On PC you can also use the medigel button.)
4) Fire again.