In the context of discussing kohanim who were not permitted to partake in the Temple service for a variety of reasons enumerated in the Mishnah (daf, or page 98b), the Gemara discusses the laws of an onen – a mourner on the day of death of a close relative – and mourning practices generally.
The Mishnah on today’s daf quotes a baraita that teaches that the day a person is first informed of the death of a close relative will be considered like the day of burial with regard to the laws of shivah – the week of severe mourning following burial – and shloshim – the thirty days after burial when the severe mourning is over, but the mourner still refrains from cutting his hair and so forth. With regard to the laws of the korban Pesach, however, it is only considered to be like the day of likkut atzamot – the day that the bones of a dead ancestor are collected – which would allow him to eat the Passover sacrifice in the evening (see Masechet Pesachim daf 92)
A melaket atzamot is someone who collects the bones of his relatives for final burial. During the Second Temple period – and for hundreds of years after that – there was a unique tradition with regard to burial. People were buried in the ground in plots that were designated as temporary resting places. After a number of years, when the flesh had decomposed and only the bones remained, they would be gathered and placed in an ossuary, a stone box, which would be interred in the family burial cave.
Although the gathering of the bones took place well after the death of the deceased, the day on which it took place was considered a day of mourning. The Gemara points out that we must be talking about a case where someone else did the actual gathering, since the person who did so would not be able to participate in the korban Pesach for a different reason – because he is tameh (=ritually defiled).