Hari Om. Pranams.
Here is my understanding -
Vibrations imply movement, energy. Just like when we disturb water, it creates waves in a pond that spread everywhere; just like when we strike a musical note, it creates sound vibrations in surrounding space; so too each thought entertained by us, sends out vibrations in universal mind.
When we sit near Mahatma or when we visit a holy place, it is charged with good, noble, pure thoughts. Our mind experiences those spiritual vibrations and we feel joy and peace. That is why Satsang uplifts our mind.
I remember in physics there used to be an experiment of "Tuning fork" to show how one object vibrating at the same natural frequency of a second object, forces that second object into vibrational motion and the result of resonance is always a large vibration. When we are at a place where many noble people and devotees are gathered we feel powerful spiritual vibrations and get benefited immensely.
Powerful pure thoughts travel very far and purify the world. Therefore a Yogi sitting in Himalaya also purifies and serves the world. Even a devotee chanting mantra is sanctifying surrounding environment by spiritual vibrations.