The "Fuse" in the game "Steal a Brainrot" refers to a game mechanic where players can combine up to four Brainrots using a Fuse Machine to create new, rare Brainrots. The Fuse Machine replaces the old Bubblegum Machine and allows players to submit multiple Brainrots, then fuse them to get a potentially more valuable or rare Brainrot in return. The chances of obtaining high-tier Brainrots depend on the rarity of those used in the fusion process. Additionally, there's a special Witch Fuse machine for a Halloween-themed event that works similarly but with unique Brainrots and chances. Key points:

• Fuse Machine lets players combine four Brainrots to get new ones.
• Rarer Brainrots submitted increase chances of rare outputs.
• The Witch Fuse is a limited-time variant for Halloween with special Brainrots.
• Players may use Robux to speed up the fusion or reveal results early.
• There are defined fusion chances based on the combination of Brainrot rarities.

This mechanic is central to progressing and collecting rare Brainrots in the game "Steal a Brainrot" on Roblox.