Menu: Code → Folding → Expand/Collapse

The goal of this feature is to let you hide things you don’t care about at the moment. In its simplest form, it will hide a whole code block (e.g. ignoring the import list when you open a new file). A more interesting use is that it will hide the boilerplate around simple anonymous inner classes and make it look like a lambda expression.

Additional tip: You can setup the default folding behaviour in the preferences at Editor → Code Folding

Shortcut: cmd+plus/minus (mac) or ctrl+shift+plus/minus (windows/linux)