A TRRS cable has two male plugs, each with four contacts: the Tip, Ring, Ring and Sleeve—which is where the acronym TRRS comes from. With split keyboards, a TRRS cable is often used to connect both halves, with both plugs being 3.5mm in diameter.
On many keyboards, only a TRRS cable can be used to connect both halves. Most 3.5mm cables only have three contacts, meaning they're TRS cables. When you use a TRRS cable with four contacts, you're most sure that they'll work with your keyboard.
The cable supplies power and data to the other half of the keyboard, so that it is able to turn on and talk to the other side of the keyboard, so that it's then able to send your keypresses from both halves to the computer.
Take care not to plug in or plug out the cable while the keyboard is powered on. The rings on the cable slide over the contacts, causing electrical shorts. While the keyboard circuitry is low power and is protected by a self-resetting fuse, it may sporadically cause damage to your keyboard. It's better to be safe than sorry - pull out the USB cable first before you disconnect both halves.
You may want to limit the length of your cable. Lengths up to a meter generally work. Longer lengths may work, but are untested. The reason for this is because the data that is sent between halves takes time to get across a longer cable, and using a lengthy cable may cause timing issues, leaving the halves unable to talk to another, which in turn makes your keyboard not work. This includes coiled cables, where the length may add up quickly.