This is very much a fluff video, marketing level, etc. But if you have never seen anything at all about the game, it shows some useful high level information.
Hopefully this embeds correctly. Is it way way too big? Anyway, it's what the game looks like during the planning phase, before you match up with an opponent to fight. This is where you build your team. I've put a bunch of red squares on the screenshot and numbered them:
1 - Bottom Left. This shows your current level, how much XP you need to get to the next level, and contains the UI buttons to buy XP and refresh your shop. Some basics: At level 4 I can put 4 units on the board. At level 5 I can put 5 units on the board. The minimum level once player combat starts is 3, the max towards the end of the game is level 9.
2 - Center Botton. This is the shop. There are 5 cards in the shop, each of them a league champion. Some things to note here: Each unit has a different cost. Gangplank, Ashe, and Nasus cost 1 gold. Camille costs 2 gold, Kai'Sa costs 3 gold. There are a pool of cards from 1 cost to 5 cost. More details on this as we go on. At the top of box 2 in the middle you see the number 4. I have 4 gold, so I can buy Kai'Sa and Ashe, or any combo up to 4 gold.
3 - Right - This is the scoreboard. The player whose name you cannot see is me. The other 7 with a name next to their score are my opponents. This is very very early in the game so four of us still have 100 health. The bottom four players lost the first round, you can see their health values of 98, 96, 96, and 94. The more units your opponent still has alive when they kill you, the more damage you take.
4 - Left - This is a list of your synergies. Back on box 2, note that Camille has two traits: "A.D.M.I.N" and "Renegade". Ashe has "LaserCorps" and "Sureshot", etc. On my current board I'm playing 3 LaserCorps and 2 Brawler. My team gets bonuses based on those synergies. This is core to the gameplay, you want to field teams that synergize well together to give yourself more power.
5 - Lower Left'ish - This is your bench. In the bottom of the red box is a bow. That's an item component. Looking ahead to box 6, you can see over some of my units are icons, those are completed items that I've put on my units. There are a set number of types of components (8 I think?). Combine any two components and you complete an item. Items give your units bonus stats and effects.
6 - Middle - This is my board. I'm level 4 so I've put 4 units on the board. The other half of the board will be filled in by my opponent when the round starts and we'll fight. I have tanky units up front, damage unit in the back. There is a ton of depth to this that we can go into later.
7 - top - We're on stage 2-2, there are 28 seconds for me to take actions before the board is locked and the fight round starts.
8 - (not really, there's a box I forgot to label, above box 2) - lower middle. There are units not on my board, but that I've purchased. This is my bench. I may want to play these units later or see which ones I can upgrade.
Whew, ok, that's a ton. Probably too much. Whee!!!