The "tube" was the BBC micro's major expansion bus. Technically, it's a high speed asynchronous, buffered parallel IPC channel with no specific purpose.
Rather neatly, you could chuck a second CPU on the end of the bus and just use the host machine for IO. That made the machines EXTREMELY fast for the time. Many second CPUs were developed with different architectures from 6502, Z80, 32016 and the original ARM CPU (which was developed as a slave of a BBC micro!). I myself have owned a 6502 unit.