Bitcoin and bank set the subject; the number 21 recalls the protocol's hard limit of 21 million coins and signals familiarity with Bitcoin's design. On .org, the same words take on a civic tone. Readers associate the ending with organisations that serve members or the public, so the address lends itself to cooperative structures, standards work, research and education around banking with Bitcoin.
Within the Bitcoinbank21 set, bitcoinbank21.org is one ending of a name series that is also held on .com, .eu and others; the whole series can be acquired together. Typical projects include a cooperative Bitcoin savings bank, an association of Bitcoin-friendly financial institutions, a consumer-protection resource, or the non-profit arm of a commercial Bitcoin bank.