It is a light that implies life and memory of love and home and earthly pleasures... something they desperately desire, but can't have anymore.
20 seconds sound clip from the Poltergeist (1982) movie soundboard.
You can hear this line at 01:19:03 in the Blu-ray version of the movie.
Quote context
[...]
- Would you all come on in? Gather round.
- There is no death. It is only a transition to a different sphere of consciousness.
- They linger in a perpetual dream state, a nightmare from which they cannot awake.
- Inside this spectral light is salvation. A window to the next plane.
[...]