what does it mean "${#pass}"

[doncidaci]

i wanna to write a script to change password for many servers

i find a function it can create random password following lines
Code:

function pwdgen{
while [ "${#pass}" -le "$passLen" ]
do
passUp="${strUp:$(($RANDOM%${#strUp})):1}"
passLow="${strLow:$(($RANDOM%${#strLow})):1}"
passNum="${strNum:$(($RANDOM%${#strNum})):1}"

pass="$pass$passUp$passLow$passNum"
done
}
what is ${#pass}? and following "${strUp:$(($RANDOM%${#strUp})):1}"
who can explain it thx for any help

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