This shows up when I startup Debian. Is this a problem, and how can I fix it?
The kernel is letting you know that your system needs additional firmware to enable all its features.
To install this firmware, you first need to enable the “non-free” repositories; as root, run
sed -i.bak 's/stretch[^ ]* main$/& contrib non-free/g' /etc/apt/sources.list
This will allow you to install the missing firmware:
apt install firmware-misc-nonfree firmware-iwlwifi
Then you need to update your initramfs: