What does "$1/*" mean in "for file in $1/*"

Question

The short bash script below list all files and dirs in given directory and its sub. What does the $1/* mean in the script? Please give me some references about it. Thanks

#!/bin/sh

list_alldir(){
    for file in $1/*
    do
        if [ -d $file ]; then
            echo $file
            list_alldir $file
        else
            echo $file
        fi
    done
}   

if [ $# -gt 0 ]; then 
    list_alldir "$1"
else
    list_alldir "."
fi

Answer 1

It's the glob of the first argument considered as a directory

In bash scripts the arguments to a file are passed into the script as $0 ( which is the script name ), then $1, $2, $3 ... To access all of them you either use their label or you use one of the group constructs. For group constructs there are $* and $@. ($* considers all of the arguments as one block where as $@ considers them delimited by $IFS)

Answer 2

$1 means the first parameter.
for file in $1/* means loop with the variable file having the value of the name of each file in the directory named in the first parameter.