Slant rhyme is a poetic technique used a lot by Dickinson in her poetic works. It is also known as a 'half rhyme,' which points towards the definition. A slant rhyme is therefore when two words are made to nearly rhyme, but not quite. It is not a full rhyme, when two words rhyme completely, but the similarity of some sounds make a partial concordance in the sound, that has the effect of producing a broken or unsatisfactory link between the two words. In the poem you have identified, you might want to think about how 'rooms' and 'names' are shown to be a slant rhyme in the final stanza:
And so, as kinsmen met a night,
We talked between the rooms,
Until the moss had reached our lips,
And covered up our names.
Note how the same 'ms' sound is present in both, but the vowel difference clearly shows that this is not a full rhyme. Thus the difference between the two corpses is highlighted by the slant rhyme.