The syntax:

VARIABLE=${1:-DEFAULTVALUE} #set VARIABLE with the value of 1st Arg to the script,
                            #If 1st arg is not entered, set it to DEFAULTVALUE


#One simple bash script code:

...
tmpdir=/tmp
defvalue=1

DIR=${1:-$tmpdir} # Defaults to /tmp dir.
VALUE=${2:-$defvalue} # Default value is 1.

echo $DIR
echo $VALUE
..


Now while running the script, specify values for both the arguments.
$ ./defvaue.sh /dev 23
/dev
23

This time don't mention their values.
$ ./defvaue.sh
/tmp
1

so

DIR=${1:-$tmpdir}
VALUE=${2:-$defvalue}

is a replacement for the following:

[ -z $1 ] && DIR="/tmp"
[ -z $2 ] && VALUE=1

http://unstableme.blogspot.com/2007/12/setting-default-value-for-shell.html

To see how we can set default values for bash script variables inside bash script

Setting default value for BASH variable

Bash

Bash