This is a multi-fasta file from your current history that provides the genome sequences for each chromosome/contig in your build.
Here is a snippet from an example multi-fasta file:
>chr1
ATTATATATAAGACCACAGAGAGAATATTTTGCCCGG...
>chr2
GGCGGCCGCGGCGATATAGAACTACTCATTATATATA...
...
The length format is two-column, separated by whitespace, of the form:
chrom/contig length of chrom/contig
For example, the first few entries of mm9.len are as follows:
chr1 197195432
chr2 181748087
chr3 159599783
chr4 155630120
chr5 152537259