Auto-removable are packages that can be automatically removed once the software that depends on them is no longer installed.
The full list is printed with:
$ apt-mark showauto
Example
You install the package scid in hope it will help improve your chess skills:
$ sudo apt-get install scid
...
The following extra packages will be installed:
fonts-lmodern libpotrace0 libptexenc1 libsnack-alsa libsynctex1 libtcl8.5 libtk-img libtk8.5 libzzip-0-13 lmodern scid-data tcl-snack tcl8.5 tcllib tdom tex-common texlive-base texlive-binaries texlive-games texlive-latex-base texlive-latex-base-doc tk8.5
Suggested packages:
libtk-img-doc toga2 phalanx glaurung crafty scid-spell-data scid-rating-data tcl-snack-doc tcl-tclreadline perl-tk
Recommended packages:
libsnack2
The following NEW packages will be installed:
fonts-lmodern libpotrace0 libptexenc1 libsnack-alsa libsynctex1 libtcl8.5 libtk-img libtk8.5 libzzip-0-13 lmodern scid scid-data tcl-snack tcl8.5 tcllib tdom tex-common texlive-base texlive-binaries texlive-games texlive-latex-base texlive-latex-base-doc tk8.5
0 upgraded, 23 newly installed, 0 to remove and 211 not upgraded.
Need to get 0 B/94.7 MB of archives.
After this operation, 227 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
You later find scid really awkward to use compared to pychess, so you uninstall it:
$ sudo apt-get purge scid
...
The following packages will be REMOVED:
scid*
0 upgraded, 0 newly installed, 1 to remove and 211 not upgraded.
After this operation, 6,695 kB disk space will be freed.
Do you want to continue? [Y/n] y
However, look at those maths: 227 MB of additional space will be used; 6,695 kB disk space will be freed. Huh?
You might want to get rid all of those dependency libraries scid depended on but are now no longer required:
$ sudo apt-get autoremove
...
The following packages will be REMOVED:
libsnack-alsa libtcl8.5 libtk-img libtk8.5 scid-data tcl-snack tcl8.5 tcllib tdom texlive-games tk8.5
0 upgraded, 0 newly installed, 11 to remove and 211 not upgraded.
After this operation, 40.5 MB disk space will be freed.
Do you want to continue? [Y/n] y
So what happened with the rest 180 MB? I have no idea. But here they are.
$ sudo apt-get purge fonts-lmodern libpotrace0 libptexenc1 libsynctex1 libtcl8.5 libtk8.5 libzzip-0-13 lmodern tcl8.5 tex-common texlive-base texlive-binaries texlive-games texlive-latex-base texlive-latex-base-doc tk8.5
...
The following packages will be REMOVED:
fonts-lmodern* libpotrace0* libptexenc1* libsynctex1* libtcl8.5* libtk8.5* libzzip-0-13* lmodern* tcl8.5* tex-common* texlive-base* texlive-binaries* texlive-games* texlive-latex-base*
texlive-latex-base-doc* tk8.5*
0 upgraded, 0 newly installed, 16 to remove and 211 not upgraded.
After this operation, 180 MB disk space will be freed.
Do you want to continue? [Y/n]
To have apt treat suggested packages as required, add
APT::Install-Suggests "true";
to one of the files in /etc/apt/apt.conf.d/.