This is one of the options you have on the individual Tiddler menu; unless you are using a very old browser it is usually represented by the symbol 'X'.

Clicking this option on a specific Tiddler menu will close that specific Tiddler: for instance, click the (X) button on this tiddler and it will be gone!

Other ways to close

You can ALSO close individual tiddlers by clicking the little X that you will see beside them in the Currently Open list on the Sidebar in the left hand margin.