A Hohmann transfer
is theoretically the most efficient way of going between two circular orbits where the orbits are in the same plane.
The basic idea is that an 'instantaneous' burn is performed to change the circular orbit into an elliptical orbit that just touches the final desired orbit.
Once the vehicle just reaches the final orbit, a second circularising burn occurs.
This orbit is only the most efficient method if no other significant body is nearby, otherwise a fuzzy orbit transfer is more efficient.
For example, a Hohmann transfer is not the most efficient way to go from the Earth to the Moon as the Moon's gravity modifies the ideal orbit away from a true Hohmann; a fuzzy transfer needs only 2/3 of the delta_v.
As a more extreme example, an orbital transfer from high earth orbit to Mars would take about 1.3 km/s delta-v using a near Hohmann trajectory (taking about 9 months), whereas a fuzzy orbit transfer takes (under ideal conditions), essentially no delta-v (but would take years, or even centuries).