A variation is a change to the works from what the contract described: more work, less work, different work, or changed conditions and constraints under which the work is done.
Standard forms set out how variations are instructed and valued, and most disputes trace back to process breaking down under pace: work changed on a conversation, instruction confirmed late or never, records of the changed work thin.
The commercial rule is simple: a variation is worth what you can evidence. The instruction (or the request and conduct that imply one), the changed work itself, its time effect and its cost all need contemporaneous support, which is why disciplined daily records and photographs are the quiet foundation of every well-recovered variation account.