@Seloronios, you can import item sets into you League Client, so that when playing with champs you have item sets already pre-made, those sets will be available at your ingame shop (you can switch sets clicking on the arrow that follows the set's name (ie. "sett top build" ⬇️.)
I think what they meant is a link for an item set. What an item set does is that it shows those specific items that you have chosen and shows them for you in-game. For more reading, please refer to this Riot Games article:
https://support-leagueoflegends.riotgames.com/hc/en-us/articles/201752904-Item-Sets
Please be sure to ask if you have any other questions :).
@Seloronios, you can import item sets into you League Client, so that when playing with champs you have item sets already pre-made, those sets will be available at your ingame shop (you can switch sets clicking on the arrow that follows the set's name (ie. "sett top build" ⬇️.)