What is the difference between insulation & absorption ?

There is often confusion between sound insulation and sound absorption.

Sound insulation is required in order to eliminate the sound path from
a source to a receiver such as between apartments in a building, or to
reduce unwanted external noise inside a concert hall. Heavy materials
like concrete tend to be the best materials for sound insulation –
doubling the mass per unit area of a wall will improve its insulation
by about 6dB. It is possible to achieve good insulation with much less
mass by instead using a double leaf partition (two separated
independent walls).

Sound absorption occurs when some or all of the incident sound energy
is either converted into heat or passes through the absorber. For this
reason good sound absorbers do not of themselves make good sound
insulators. Although insulation and absorption are different concepts,
there are many instances where the use of sound absorbers will improve
insulation. However absorption should not be the primary means of
achieving good sound insulation.