What this means is that you will be able to download the game in its entirety, and upon its release, which is set for the 28th of January, you will be able to launch it and get playing right away. This is versus waiting for the 28th of January to download it, in which the time spent downloading could be the time spent playing the game.
Also sometimes depending on the popularity of the game, servers might be congested during launch, meaning that speeds could be slower and you could run into issues. In any case if you have some time to spare, why not preload the game so that you can start playing it at launch, right?
If you haven’t bought the game yet, there is still time to do so. You can find the minimum PC requirements here. PS4 gamers on the other hand will still have to wait until the later part of 2016 before they can get their hands on it.