What does 2 commas after variable name mean in bash?

This is called “Parameter Expansion” available in bash version 4+ . To change the case of the string stored in the variable to lower case.Eg:

var=HeyThere
echo ${var,,}
heythere

You may want to try some additional commands and check the effect : source

${var^}
${var^^}
${var,}
${var,,}

Note: “Parameter Expansion” is present in man bash .Search for it.