Time Travelling is when you jump back and/or forward in time within the game (rather than playing it in real-time). It helps you collect things like bugs/fish as you can change the season to when specific ones appear (e.g. String Fish during winter). It also helps with filling the catalog as you can skip through days quickly in order to change the shops daily stock.
You can use the 'I need some help first option" before you load your character (there should be a "change date/time" option but it is better to change the 3DS system clock). before you even start your AC;NL game you go Settings>Other Settings>Date and Time and change the date/time according to what it is you want to do, then exit settings and start your game as normal.
I hope this helps, I TT a lot so I thought I should try answering your question.