Pressing L, R, or Z seven frames before landing with an aerial halves your landing lag. Fox benefits tremendously from this; most of his aerials become safe-on-shield and can lead into true follow-ups, most notably shine and up-tilt.
L-cancelling is challenging; not even top players have full consistency. Down-air (“drill”) is particularly hard to L-cancel, because it can hit a dozen times or miss completely and opponents can SDI (ASDI) it in unpredictable ways.
L-cancelling is one of the few techniques that is always beneficial to perform. It cannot be countered by your opponent in any way but makes you move and attack faster with no drawbacks.
It is technically optimal to L-cancel with either Z or analog presses of L/R. This will prevent activating the tech-lockout window, so even if you get hit out of your aerial, you will be able to tech.