The burden is the five aggregates of attachment (upādānakkhandha), the burden-bearer is the person (puggala), the taking up of the burden is the lust that leads to rebirth, the laying down of the burden is passionless ceasing of craving. S.iii.25 f.
See A Discourse on the Bhāra Sutta.
Bhāra Vagga.– The third chapter of the Khandha Saṃyutta. S.iii.25‑33.