Semi-sticky means that it re-appears when scrolling back up. A sticky or semi-sticky header makes visitors not have to scroll all the way up to go to another page or do another search.
You can also simplify the sticky header that remains up as they scroll down or the semi-sticky header that shows up as they scroll back up. For example, the simplified header when they’re anywhere except at the top of page could be the burger menu, search box and cart icon.
And then you can show the full header when they’re back all the way up to the top of page.
Space is obviously limited on mobile, so if your header has 2-3 rows, it doesn’t make sense to keep it all visible at all times. Reducing it to a single thin row with only the crucial features leaves more space for the page contents.