Short answer: Plumbing installation covers putting in new faucets, toilets, garbage disposals, water heaters, or other fixtures and connecting them properly to existing supply and drain lines.
Installation is usually a planned project rather than a reaction to something breaking — a remodel, an upgrade, or adding a fixture that wasn't there before. Getting the connections right the first time matters, since a poorly sealed installation can lead to leaks down the road.
The pages below cover installation by fixture type.