False means, do not show. True mean, show/visible. its like If statement of any programming language.
Have a look at next blog post (Ignore first 3 screenshots)
In your case, I will recommend, you to have two separate panes/tablix; one for summary report and other for Detailed report. Both should be inside body of report. and you can Hide whole Tablix based on condition. There is no need to put expression on individual textboxes, when you are hiding whole tablix.
The purpose to keep both Tablix inside body of report, is to save whitespace, so hiding elements do not result in whitespace.