No, a water hammer is not a piece of heavy machinery or a new-fangled plumbing tool. It is actually a sound, and a very loud one. When the water flow in the pipes is suddenly shut off, the shock wave (from the shift in water velocity) produces a very distinct knocking or hammering sound, or a rumbling or shaking.
Water hammers are actually quite common. People will hear it when the clothes washer or sprinkler suddenly stops or switches to another mode. Dishwashers can also produce the sound when it changes cycles. Even faucets will produce water hammers if suddenly turned.
Water hammers may be common, but these should also be avoided. The pressure can eventually cause the joints, valves and pipes to weaken or rupture. Frequent water hammers can also be a sign of a malfunction that should be addressed before it gets worse and becomes more expensive to repair.
For example, water hammers may reveal that the pipes are the wrong size, or that the system lacks a proper pressure-reducing valve. It may also indicate a need to add “bends” to the pipes, or to reinforce the piping system structure. Older homes, in particular, may have a lot of water hammers because of waterlogged air chambers, and may need a plumber to drain the system and allow risers to refill with air. However, it is better to replace air chambers altogether, which come with water hammer arrestors. Note that it’s important to get the right-sized parts to prevent leaks or other potential plumbing problems.