Meaning of obj.prefetch

[andan andan]

Hi.

The obj.prefecth variable, What is it supposed to do?

The default VCL set this variable to -30s, and varnishlog shows (30
seconds before that object expires):

    0 ExpPick      - 964711497 prefetch
    0 VCL_call     - prefetch
    0 VCL_return   - fetch
    0 Debug        - "Attempt Prefetch 964711497"

Can someone please, explain what does it mean.

Thanks in advance.
BR.