'Gu' means dark and 'ru' means light. A guru is somebody who leads us from darkness to light. In practical life, it is much easier to learn meditation with a guru than to do it alone. A guru not only offers outer instructions but can also lead and guide the student inwardly.
Most gurus have themselves learned from a guru. In the tradition there are whole lines of gurus who pass on the knowledge over generations.
Not everybody who teaches meditation is automatically a guru or a spiritual master. Most meditation teachers are still seeking the highest truth in their own lives but willing to share with other seekers what they have already learnt in their own personal journey of self-discovery. But real gurus are expected to have reached the ultimate goal, the self-realisation, and with it the capacity to know the truth directly and to guide seekers to this truth. This kind of master is very rare.