This question is rather debated within the Pit community. Personally, I believe wing refreshing is the most useful technique, allowing for more aerial freedom to edgeguard and perform various other techniques.
It is performed by coming in contact with the ground in the first two seconds of WoI, and then continuing your flight. Before the WoI time runs out, you must cancel out of WoI by performing an aerial attack. Afterwards, you have all of your jump options back, as well as another WoI option. If you are hit during the initial WoI (after touching the ground), you will still have your jump options available.
The easiest way to perform this is to WoI immediately after jumping. While it is possible to WoI straight off of the ground, fall down, touch the ground, and then fly back up, this takes time and is prone to autocanceling.
Before learning ATs, though, make sure you have a solid grip on Pit's BASIC gameplay. ATs are to support your abilities, not to be the focus of them.