It is recommended to also look at u/Whitesushii's affiliated visual guide which can be found in the FAQ/Guides section of the subreddit. This post assumes you have taken a look at that guide already. It provides in-depth information about the version of the event from May 2020 that had martial ratings. On top of the general information it provides, the guide is focused towards mid-game players.
This post provides some quick general notes about what in the guide does or does not apply in general to the recurring event.
Heroes of Esperia is a recurring event and although the aforementioned guide is specific to the May 2020 version of the event, it covers some of the general information.
The Heroes of Esperia event can occur with or without martial ratings. Martial ratings adjust the level of all participating players' heroes to 240 + (Current resonating crystal level/10 rounded down). Gear is also adjusted to the same level and retains its faction bonus if it is Mythic. Elder Tree levels are maintained and can provide large advantages.
Free attack attempts are given every day and more can be bought with diamonds. Winning or losing attacks increases or decreases your number of trophies respectively. The number of trophies determines your rank (Legend > Master > Diamond 1-3 > Gold 1-5 > Silver 1-5 > Bronze 1-5). Once you reach 10,000 trophies you enter Master where you can attack the lower 200 of Legends players who have 10,001 trophies. Once in Legends, it is a pure ranking system.
From Bronze to Gold, 3 teams are used. Beyond this, 5 teams are used in Diamond until Masters where 7 teams are required. Winning fights requires winning a majority of the team on team fights.
Martial ratings make it possible for many mid to late game players to enter Masters as this version of the event is focused more on strategy and dedication (see how to do this in the aforementioned guide). The version of the event without martial ratings is based on pure hero power as the levels are not adjusted until Legends (where they are normalized to 480) thus making it so only end-game players able to field greater numbers of strong, high-level heroes can make it this high.
The rewards are given based on rank at the end of the event and provide scrolls and points to earn frames.
Hope this helps!
Credits to u/zeedafluff.