Everything is energy, and all energy vibrates at different levels. Vibrational frequency is the rate at which the atoms and sub particles of a being or an object vibrate. The higher this vibrational frequency is, the closer it is to the frequency to light.
When in a lower vibration you have a hard time grounding and manifesting your reality, tend to become ill, have judgements of ourselves and others.Self sabotage, focus more on the negative than positive, are needy or demanding of other people, argumentative, controlling, overly cynical and skeptical, struggle with apathy etc.
Have you ever walked up to a person or went into a place and felt that it has really good energy or has it made you feel uncomfortable? You are feeling the vibration, the energy that permeates the person, place or thing.