“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 together with a Guru than to do it alone. A Guru does not only give 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 just one step ahead of normal people. 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.