What does it mean to dream about hammer?
A hammer usually stands for force put to work, building or breaking, so notice whether you used it to make, to mend, or to strike out.
Four ways to read it
Each reading below comes from a named tradition and a cited source, so you can see where it came from and decide what fits your dream.
Freud
Freud would read the hammer as a tool of force and assertion, bound up with power and at times aggression. Wielding it can express the wish to make an impact or to strike at what stands in the way.
After Freud, The Interpretation of Dreams (Brill trans., 1913), public domain.Jung
Jung might see the hammer as the force that shapes and forges, the will that builds the self or breaks down the old. Whether it makes or destroys can show how you are using your strength.
Jungian concept (our summary).Modern
Threat-simulation theory reads a hammer raised as a weapon as the sleeping mind rehearsing danger and defence. As a tool, continuity ties it instead to waking efforts to build, fix, or force a result.
Threat-simulation theory (our summary).Mythological
The smith's hammer forged the wonders of myth, and the thunder god's hammer both struck down giants and hallowed what it touched. The tool of the forge was at once a weapon and a maker of marvels.
Rosmerta Publishing editorial synthesis (after Bulfinch; Brewer).