A method statement describes how a specific piece of work will be carried out: the sequence of operations, the plant and equipment, the workforce and supervision, and the controls that manage the risks identified in the accompanying risk assessment.
It is written by the party doing the work and reviewed by the principal contractor, then briefed to the operatives carrying it out. Its value on site is practical: everyone doing the task knows the agreed sequence and controls.
When methods change, the statement should be revised and re-briefed; the trail of versions and briefings becomes important evidence if anything goes wrong.