Str_contraction in Mutect2 output

Question

[Raman2]

What exactly are str_contraction in the mutect2 output and what is their significance? The description says "Site filtered due to contraction of short tandem repeat region". Does this mean there was a mutation (snp or indel) in the STR region? What is the difference between a STR mutation call which has PASSed vs filtered due to str_contraction

Thanks!