Contiguous means being adjacent. In this situation, it is the number of residency programs within the same specialty in a row that you rank before you rank a program within a different specialty.
IM
Rank 1: program A
Rank 2: program Y
Rank 3 program H
Dermatology:
Rank 1: program 3
Rank 2: program 5
Rank 3: program 9