‘Bhaja Govinda’, on one hand, means ‘to chant the name of Govinda’. Now ‘bhajan’ is not just chanting, it also means to completely absorb oneself in something.
Why is it called ‘bhajan’ when songs are usually called ‘geetham’? This is because ‘bhajan’ has to do with inner absorption. It is actually the spiritual version of ‘bhojan’ (meaning food) for the physical body. It is a ‘bhojan’ for the mind.