Could someone please help explain to me what '收尸' means?
Like dan said, 收尸 is a verb, which means 'collecting (a) dead body'.

The example in question only makes sense if you knew who Pharod Wormhair is. Basically he is someone who makes money from dead bodies, and he has his collectors, who collect dead bodies for him. Here is an introduction of him if you want to know more.
Here 收尸人 is used to describe his collectors. The translation from 'collectors' to 收尸人 is only specific to this context, otherwise 'collector' doesn't directly translate to this.

Normally, the word 收尸 is used between people who are relatives/very close.
For example, we have A, whose only acquaintance in the world is B. Then B dies. Now we say, A has no one to collect their body.

Ask if you're still unsure about this word.