It’s the starting index for the search, the same as the start argument of findnext — it says to search starting at byte 1. Passing 2 means to search starting at byte 2.
In this way, you can find multiple occurrences:
i = Base._searchindex(a, [0x90,0xea,0x00], 1)
Base._searchindex(a, [0x90,0xea,0x00], i+1) # finds the second occurrence, if any