Image thumbnail hovering

First of all excuse me for doubleposting my issue. (yesterday i wrote the same issue but in the wrong place of this forum. )?

Basically the idea is that i have an image thumbnail created by custom fields, and i wuold like to show a 50%black box with the post title and a little description onto the thumbnail once i hover it.
I spent a lot of time trying to wonder how, but any consistent idea…. ??

Thanks in advance!