bash scription if conditional "${1:-}" [duplicate]

I try to understant the if condition

log_daemon_msg () {
    if [ -z "${1:-}" ]; then
        return 1
    fi
    log_daemon_msg_pre "$@"

    if [ -z "${2:-}" ]; then
        echo -n "$1:" || true
        return
    fi

    echo -n "$1: $2" || true
    log_daemon_msg_post "$@"
}

what is mean "${1:-}" and "${2:-}"