A riser is a supply line, usually made of metal or plastic that is intended to connect a faucet or shower fixture to the stop valve of the water supply. Plumbing risers are commonly made of copper. Metal flex risers, however, are actually corrugated, which allows them to be bendable.
The term "riser" can also be used within the context of plumbing to describe the supply line that rises from one story to the next. These kinds of risers can be frequently found in structures that are multiple stories because they require plumbing that can be facilitated through multiple stories of structure, so the supply line will commonly have sections of risers in order to facilitate this.