The historical claim attached to this company is usually stated loosely, so it is worth stating exactly. On 27 November 2010 a Bitcointalk user posting as slush opened a thread proposing that small miners pool their work: "Join poor CPU miners to one cluster and increase their chance to find a block!" The service that followed was not called Slush Pool. Braiins' own history says Marek Palatinus launched it as the Bitcoin.cz Mining Pool, and that it "officially rebranded to Slush Pool" only in 2016, by which point the community had been calling it that for years. It became Braiins Pool in 2022.
Braiins itself is a separate story that later merged with the pool's. Jan Čapek and Pavel Moravec started an embedded software company in Prague in 2011. The Czech commercial register records Braiins Systems s.r.o., registration number 24147931, entered on 1 September 2011 at the Municipal Court in Prague under file C 183056, with both men as its first managing directors. By the company's account they took over development and operation of the pool and acquired a majority share in 2013, and bought the remaining stake from Palatinus in 2020. The pool service is today operated by a separate entity, Braiins Mining Limited, registered in the British Virgin Islands under company number 2038702.
The work has widened well past running a pool. Braiins publishes Braiins OS, replacement firmware for mining hardware, along with fleet management software and a hardware line, and it is a co-author of Stratum V2, a rewrite of the protocol between miners and pools. By Braiins' account it "was developed in 2019 by the Braiins founders Pavel Moravec and Jan Čapek, in collaboration with Matt Corallo and other industry experts". The part that matters is block construction: Stratum V2 "gives miners the power to choose which transactions they include in a block". That moves a decision about transaction censorship away from a small number of pool operators, which is the concentration risk pooled mining created in the first place.