Question: What is the meaning of ‘form is emptiness’ (色即是空)?
Answer: ‘Form’ (色) refers to matter, (which is what has mass); the material, as summed by the Four Elements (四大) of earth, water, fire and wind (地水火风), which is shorthand for all physical elements in all their states. Form can also become energy interchangeably, as E=mc squared, with ‘E’ being energy, ‘m’ being mass (and ‘c’ being the speed of light).
‘Emptiness’ (空) does not refer to ‘nothingness’ (空无所有); it refers to being empty of the unchanging (i.e. permanent: 常) and substantial (i.e. that with fixed unchanging self-nature: 我). Forms are still there, but not in unchanging and substantial ways; but as impermanence (无常) and non-self (无我).
As all forms are changing and unsubstantial, thus, ‘Form is Emptiness’. For more profound details on the Heart Sutra, which is the source of the teaching, do look out for the next run of this course: https://thedailyenlightenment.com/2016/12/the-heart-of-the-heart-sutra-the-core-of-wisdom-11th-run < This is a previous run.