Initially, the Russians did not wish to launch the FGB (Functional Block), now named Zarya, which was the first ISS component launched. But the US wanted it, so they could launch a US component sooner. That is, the FGB could provide control, life support, storage, a Canadarm grapple point, so that the first US node could be immediately attached, rather than waiting for the Service Module to be ready (Zvezda). Thus, the US sort of funded it. (The complete story appears to be very complicated, so saying the US paid for it, is reasonably close to the end result).
There is a docking 'ball' (module? Compartment?) at the front end that has a PMA (Pressurized Mating Adapter) linking it to the CBM port on Node 1 (Unity).
Zvezda was built with 6 ports (one of course attached to Zvezda itself) and reduced down to 4 total (Up, down, one to itself, and one to dock to Zarya). With Pirs, and Poisk consuming the ports, and Pirs is being discarded to be replaced with the MRM Nakua.
But Zarya only came with one, now Rassvet, that MLM-1 is docked to it.
Why did they not supply more ports? (Similar to the question Why were the docking ports removed from Zvezda but for a different module).