"Random Crits Are Fair and Balanced" is a sarcastic inside joke that comes from the online multiplayer first-person shooter Team Fortress 2.

Technical explanation
It is related to a game mechanic called: "Random Critical Hits" (Or Random Crits).
Sometimes, when you shoot with your primary, secondary or melee weapon, the game will give you a random critical hit, which deals 3 times the normal damage of your weapon. (Melee Crits rate is notorious for being very high, and one of the only reasons you would pull out your melee weapon in a fight, to get a chance to kill your enemy instantly.)

The Crit rate begins at 2% for normal weapons, and the more you do damage in the last 20 seconds, the more the rate is augmenting. (The maximum rate is 10%)

(Here is the visual effect when you get a Random Crit that hits someone.)

Meme explanation
Random Crits are a very well known mechanic in the game, and almost everyone (except new players) know about it.
When players get killed by a Random Crit, the majority of them will feel that cheated, and they will probably say [In the in-game text chat] "Random Crits Are Fair and Balanced", as a joke, to justify how broken the mechanic is. A lot of people bring out the Negativity Bias as a way to explain why people will remember that they got killed by a Random Crit more than they killed someone, giving a bad reputation to Random Crits because people will "only" talk about the negative side of them.
The meme is so popular that even some people put it as a bind in-game. (A way of telling the game to do an action when pressing a key, such as: writing something in the text chat)

Many Youtubers, such as Uncle Dane, explained why we should remove Random Crits from the game (Watch the video here), using statistics and logic to prove his point. (Which not a lot of people did, and instead explained it from a personal opinion)

Here is a compilation of random crits, and showing how the mechanic is broken, and that you "get the candy without the effort".
(If you aren't sure how random crits can be found, they sound like some sort of "electric" explosion)

Origin of the meme

The meme wasn't really "created" until 2015, when Team Fortress 2's "Gun Mettle" update released, when people started really discussing how Random Crits affected the game, either by spamming "Random Crits are Fair and Balanced" in the in-game text chat, or via Reddit and Steam Community posts.
Even if many years have passed, and many memes should've died by now, TF2 memes like "Pootis", "Nope.avi" and "Random Crits Are Fair and Balanced" are still growing strong and steadily.

(Here is a result from Google Trends, with "random crits" as the search since around 2016.)