What does 21q21.1-q21.2 mean?
Cytogenetic map notation uses the following form:
A[p/q]BC.D
Where A is the chromosome number, p is the petit / short arm, q is the queue / long arm, B is the band, C is the sub-band, and D is the sub-sub-band.
So, 21q21.1-q21.2 designates a region on the long arm of chromosome 21, band 2, sub-band 1, between sub-sub-bands 1 and 2.
Source:  NCBI Genetics Review (archived version)