What does the colon dash ":-" mean in bash

The result is the one desired; after a bit of trial and error. I don't understand what the "2:-" and "3:-" do/mean. Can someone explain.

#!/bin/bash
pid=$(ps -ef | grep java | awk ' NR ==1 {print $2}')

count=${2:-30}  # defaults to 30 times
delay=${3:-10} # defaults to 10 second
mkdir $(date +"%y%m%d")
folder=$(date +"%y%m%d")
while [ $count -gt 0 ]
do
    jstack $pid >./"$folder"/jstack.$(date +%H%M%S.%N)
    sleep $delay
    let count--
    echo -n "."
done

It's a parameter expansion, it means if the third argument is null or unset, replace it with what's after :-

$ x=
$ echo ${x:-1}
1
$ echo $x

$

There's also another similar PE that assign the value if the variable is null:

$ x=
$ echo ${x:=1}
1
$ echo $x
1

Check http://wiki.bash-hackers.org/syntax/pe