For example. At the top where the menus are and it says -Applications, Places, System.
If the mouse goes over the menu always this tool tips show up.
Also if a menu is open any entry in there has more tool tips.
Pretty much anything on the desktop comes up with this (black + white) tool tips.
There are quite a few infos on the net how to turn the tool tips off but it seems for me
that this infos are outdated and do not work anymore.
I was able to disable them in 16.04 but in 17.04 not anymore.
Use dconf Editor. org-> mate-> panel-> general-> tooltips-enabled toggle to false. I use a custom Marco compositor and the tooltips that do show up (bottom panel) have a nice light yellow background and plain text.