Hot Corner is a Gnome Feature that allows you to see all open windows (same as clicking Super key or clicking on Activites) by moving mouse to top left corner.
To see if hot corner is enabled or disabled, run
gsettings get org.gnome.shell enable-hot-corners
To enable hot corner, run
gsettings set org.gnome.shell enable-hot-corners true
EDIT: After using enable hot corner for a day, i found my mouse reach this corner accidetly multiple times. Seeing activites page unexpectedly when you want someting else is confusing. For Ubuntu, it is better to keep it disabled.
To disable, run
gsettings set org.gnome.shell enable-hot-corners false
See Ubuntu 18.04