Overload Monsters are a new type of Effect Monsters (sometimes unofficialy called "Category monsters") created by Feder373. These are monsters which generally have higher power than it is common for their Level, like higher stats and/or more powerful effects with little to no restrictions, in exchange of having a clause that makes you lose a set amount of Life Points when they are Summoned, though the amount of Life Points lost will be recovered during your next Standby Phase.
The concept of this mechanic is that in exchange of getting an inmediate advantage you are put in a more vulnerable position just after playing these monsters, by allowing your opponent the chance to defeat you easier and limiting you in the use of other cards that involve Life Point loses (like payments or effect damage) while your Life Points are lower, especially in the mid and late game, but not making this a deciding factor in the Duel in the long-term due to you recovering the Life Points lost. This mechanic can be a deciding factor in a gamestate where OTK and FTK Decks are abundant though.
All Overload Monsters have the following clause: "If this card is Summoned: Lose X Life Points. During your next Standby Phase: Gain Life Points equal to that amount lost (even if this card leaves the field). The use of losing Life Points instead of paying or taking damage is so the drawback of the mechanic isn't overturned or even turned into an advantage by cards and effects that cancel payments or effect damage. This clause is an effect and can still be negated by cards such as "", thus making a lot of this monsters a very good choice for Skill Drain Beatdown Decks, as they will be great beaters with no drawbacks.
In fan fiction, the mechanic of these monsters is represented as the duelist losing energy through a stream of crimson light that goes from the chest of the player to the body of the monster, who absorbs it. Then, when the player regains the Life Points lost, the stream goes back from the monster to the duelist, this time in a light blue color, and the duelist recovers the lost energy.