The world menu is what you get when you click in a Project window if you click outside any morph in it (if you click inside a morph that's what you trigger).
Here are some brief comments on this list of options.
The first four options deal with existing projects. 'previous' goes to the one you left last, and 'jump' pops up a list of all projects in the image. 'save' saves a project on disk, an 'load' can load projects from disk. (There are many projects available made by others. See e.g. Bob's Superswiki and Useful Internet links.)
'can't undo' is the alternative for 'undo', which when it is in the World menu means that clicking it will undo the last recorded action you did.
'restore display' is useful to press when you have a display with lines, or parts that shouldn't be there.
The options 'open...' to 'do...' all pop up menus that can be read about from Menus.
'objects (o)' pops up the Objects tool, which contains a lot of ready made objects in Squeak you can use, enjoy, dislike, rewrite or extend.
'new morp...' up to 'debug...' are again in the list Menus.
'save' saves the image in the state it is.
TIP: Safe often, especially before or after making an
important change. Once an image has been saved you know your work has been saved
and can be returned to in just the state it was saved.
'save as...' enables you saving the image under a new name. Of course the old image will be there as well, in the state it was last saved in.
'save as new version' saves the image with a number Squeak adds from 1 onwards. This is convenient if you want to make sure an important image is saved and you want to continue in it without upsetting the work you did.
'save and quit' first saves the image and then quits Squeak.
'quit' merely quits Squeak, though you are asked whether you want to quit without saving.