(noun)
a structural element of something that is intended to provide support or reduce friction in the usage of the thing;
e.g. This tool has a bolster to strengthen the joint between the blade and the handle.
(noun)
a long, thick cushion or pillow that is used to support other pillows by being placed underneath them;
e.g. There was a cylindrical bolster on each end of the couch.
(verb)
to physically reinforce or support something, with or as if with a long pillow (i.e. with a bolster), especially to provide padded support to a seat;
e.g. He added cushions to the couch to bolster his sore back.