If more content is placed inside a frame than the Minimum Height, then the frame height will increase as normal, to accommodate the all of the content. However, if there is less content than the Minimum Height the frame will remain at the minimum height and will not decrease in height.
The most frequent use of Minimum Height is the banner at the top of your internal website pages. It is often a nice feature of your website design to have a consistent banner height throughout your entire website.
You would like all internal banner images of your website to be set at 600px high. You know that some banners will have more text in the headline than others. Setting the Minimum Height to 600px high across the entire website will ensure that your banner images never reduce in size, regardless of how little text in the headline.