What are backports?

Debian has a really good write up here on what backports are. Copying directly from their introduction paragraph:

You are running Debian stable, because you prefer the Debian stable tree. It runs great, there is just one problem: the software is a little bit outdated compared to other distributions. This is where backports come in.

Backports are packages taken from the next Debian release (called "testing"), adjusted and recompiled for usage on Debian stable. Because the package is also present in the next Debian release, you can easily upgrade your stable+backports system once the next Debian release comes out. (In a few cases, usually for security updates, backports are also created from the Debian unstable distribution.)

Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care!

It is therefore recommended to only select single backported packages that fit your needs, and not use all available backports.

Once I enable backports will all packages use them?

No! Any new packages and updates to existing stable packages will prefer the stable releases. The only time you will leverage a new backport package is if you explicitly specify to pull from them.

How do I enable backports?

First you need to add the new backport source to your sources.list file. Edit the file in vi:

sudo vi /etc/apt/sources.list

Arrow down to the last row, press o to create a new line and then enter the following:

deb http://deb.debian.org/debian buster-backports main

Press escape and then type :wq to save the changes and exit via.

Next, we need to specify a keyserver to verify the authenticity of these packages. Note we use Ubuntu's key servers to validate the packages. Interestingly, Debian has a keyring to validate the packages, however the keyring doesn't contain the backports for buster on the raspberry pi at time of writing this. Ubuntu's servers will work fine to validate the authenticity of these packages and you will ultimately pull the packages from Debian rather than Ubuntu.

sudo bash
gpg --keyserver keyserver.ubuntu.com --recv-keys 04EE7237B7D453EC
gpg --keyserver keyserver.ubuntu.com --recv-keys 648ACFD622F3D138

gpg --export 04EE7237B7D453EC | sudo apt-key add -
gpg --export 648ACFD622F3D138 | sudo apt-key add -
exit

How do I obtain a package from backport?

You can leverage one of the follow formats to specify the backport package:

apt install <package>/buster-backports
apt-get install <package>/buster-backports

or

apt install -t buster-backports <package>
apt-get install -t buster-backports <package>

or

aptitude install <package>/buster-backports