Why is this called “the first resurrection”?

The term resurrection refers to the coming to life of someone who had died. The addition of the ordinal first can lead us in two divergent directions:

1. The use of the word first opens up the possibility of multiple resurrections. However, we need to note two concerns: a) This is the only place in the Bible that we come across the phrase first resurrection; b) Nowhere in Scripture do we read of a second resurrection, let alone a third. So we ought not to insist that the concept of multiple resurrections is necessarily in view in this passage.

2. Another option is to understand the term first as initial, so as to stress its temporary nature and its promise of completion.

The latter understanding fits better in the context of Revelation 20; that is, these beheaded martyrs receive a unique form of existence (they are alive!) in heaven for the duration of the new dispensation (= 1000 years). On the Last Day, their bodies will arise from their graves (completed resurrection), be glorified, and reunited with their souls.