Save.tf has the full rundown. But basically, if you join TF2's casual mode (TF2's most popular game mode), there's a 50/50 chance the server will be overrun with user-created bots. These bots have aimbot, pick sniper, and instantly kill anyone who comes into their line of sight. They also try to votekick real players, and if there are enough bots in the match, usually succeed. The bot hosters are trolls who just want to ruin the game for everyone, and have gone as far as swatting the lead developer of MegaAntiCheat, a user-created anti-cheat for TF2.
There's been a big #FixTF2 / #SaveTF2 social media push recently, trying to push Valve to do anything to try to solve the bot problem. After a day or two of getting no response from Valve, people resorted to review-bombing the game.
Nothing. People want Valve to address issues in TF2 and have organized a review bomb campaign to get attention.
My two cents, I don't think what they are asking for is feasible. There is an organized open source cheating project as well as someone hosting cheat bots that ruin matmaking.