I use the bwa-mem to align the reads to reference genome. The official website says the parameter -k INT represents Minimum seed length. Matches shorter than INT will be missed. The alignment speed is usually insensitive to this value unless it significantly deviates 20. 
But, I was very confused about this parameter. What is the seed? Whether the seed is the length of reads exactly matching to reference? For the reads length of 150 bp, why the -k could be set to 10000000? What is the meanings of 10000000 there?
Any suggestions would be appreciated.