[TYPO3-english] question on TSref manual

[typo3 study]

Hi,

This example chooses between two different renderings of some content
depending on whether the field "layout" is "1" or not ("default"). The
result is in either case wrapped with "|<br />". If the field "header"
turns out not to be set ("false") an empty string is returned anyway.
stuff = CASE
stuff.key.field = layout
stuff.if.isTrue.field = header
stuff.stdWrap.wrap = |<br />
stuff.default = TEXT
stuff.default { .... }
stuff.1 = TEXT
stuff.1 { .... }

My question is:
what does this line mean: stuff.if.isTrue.field = header  ?

Tom