Add a "non-free" component to /etc/apt/sources.list, for example:
# Debian 9 "Stretch"
deb http://httpredir.debian.org/debian/ stretch main contrib non-free
Update the list of available packages and install the firmware-iwlwifi package:
# apt-get update && apt-get install firmware-iwlwifi
As the iwlwifi module is automatically loaded for supported devices, reinsert this module to access installed firmware:
# modprobe -r iwlwifi ; modprobe iwlwifi
Configure your wireless interface as appropriate.