Satsang is a sanskrit word. Sat means Truth and the sangha is the group of lovers of Truth.
In yogic terms, Truth refers to the ultimate truth of the universe - realisation of the higher Self that is common to all things, the realisation that we are one with the universe.
So satsang is an informal gathering of people, mostly to discuss the Truth and importance of yogic and other principles in life, and to impart this valuable knowledge verbally.
Usually the speaker (the guru or another learned person) speaks on a topic, then others may ask questions for further clarification. The speaker tries to answer each question as fully as possible.
It may also be a time where painful things (suffering, confusion) can be shared with others and left behind, so that people are not burdened with needless suffering and they can continue the journey of life with freshness.