The toilet supply line supplies cold water to the toilet tank. The supply line is arguably the most important part of a toilet, as without it the toilet would have no access to water to flush, cycle, or refill. The supply line is the small metal piece attached to both the toilet and your wall, as it runs water into the toilet from a small metal tube and pulls water from the interior wall water lines, allowing it to be shut off or turned on with a small valve handle.
The supply line is notorious for having a variety of issues. A leaky supply valve can cause water to leak inside the wall or outside that wall onto the bathroom floor. Toilet supply lines come in a variety of sizes. However, to replace a broken or leaky valve, universal hoses and adapters can be purchased, which can be shaped to fit almost any valve size.