Basically for that day your novel will appear in the stated section of the explore tab. Just means that users will easily be able to find your book as soon as they open that section as a recommended read.
You need prepare nothing, though it is advised that if you are unhappy with your current cover art and synopsis then now might be the time to switch them out, being eye catching and easy to understand from a small blurb never hurts but it's not required.
As for how big an effect it is, depends on your book. The big thing I took away from my own time recommended and seeing Regnov show his own recommendation is that like him, shortly after this advertisement I was offered a contract with webnovel, so your book could be looking at being extended an offer in the near future.
Keep in mind this last bit is just a guess and not a guarantee though, so refreshing yourself on what you want the future to be is also something to think about in case this series of events does play out like I surmise but don't take it as set in stone.