A warm welcome to Mac-Forums.
Depends exactly what you mean:
For example, in Safari, if you Click on View (in the Menu Bar) and Choose "Hide Favourites Bar", it will hide it. I don't know if that's what you're after.
Bookmarks are not visible when a Safari window is open, however, so I'm not sure what you mean by "Hide" in that context. Unless you are referring to the Sidebar which can show Bookmarks. If that is your problem: 2 ways to hide it - (1) Click on Bookmarks and choose "Hide Bookmarks" or (2) Just under the Menu Bar, level with the red, amber and green dots, and to the right of them, is an icon (sort of like a window with a sidepiece) - click on that and the sidebar disappears (Click again & it reappears).
Any help?
Ian