Auto-removable are packages that can be automatically removed once the software that depends on them is no longer installed.
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/.