The bigBed files in this directory represent highly conserved genomic regions which have undergone purifying selection over evolutionary time.
They were generated from Multiple Sequence Alignments (MSA) using the GERP software:
http://mendel.stanford.edu/SidowLab/downloads/gerp/

Information on the current set of Ensembl MSA and the species used to generate them can be found here:
http://www.ensembl.org/info/docs/compara/analyses.html

Each BED file represents the genomic coordinates of the conserved regions for a selected species (indicated in the file title).