The Loading Screen is a pre-game screen where summoners wait for everyone's game to be ready to start.
You can only see Champion Mastery and player rank of players in your own team. The Mastery 5 emblem is the red badge on the upper-left of the image here. This mastery emblem will appear if the player has at least Mastery Level 4 on that champion. Some summoners that have different borders show that their rank in the previous season was at least Silver. The border's color shows the rank, and the more the detail in the bottom part of it, the higher the division in the tier.
The summoner can also see their own ping, located at the bottom right corner of the loading screen (image is outdated). So while the loading screen is active, they can solve problems before the game starts. This is the green number 43 and dot on the bottom-right of the image here. An extremely poor ping (from 300 and above) will show as orange. When the player loses all connection to Internet, their ping will always show as an orange 999 ping. Even if the ping exceeds 999 ms at any point, it will be capped to 999 in the screen (does not affect actual connection).
The loading progress of each summoner can be shown on the top right corner of the bottom border of each summoner card as a percentage, and is visible to all players on both teams.
Starting with the July 2017 changes to honor, a player who has received honors from at least one non-premade player of their team in the previous game will have an honor icon in the top center if their Honor level is at least 3 (not shown in the image).
During the loading screen a player can use their computer even if the loading screen is on full screen. The user can use
Alt + Tab on Windows to move to other windows, or
Alt + Esc to minimize the loading screen, bringing the user back to the previous windows, if any. If there's not any other window, the user can use
Windows + D to get to the desktop.