Oops, shouldve probably explained it in the post.
ssd= server side decorations
server side decorations is when the system draws the titlebar, as opposed to client side decorations (csd) where the applications draws it.
for example, gnome uses csd, but kde uses ssd.
There’s debate over which is better, but the main difference is that csd allows applications to put anything in the titlebar, which can make the design nicer, and ssd allows the titlebars on all apps to be consistent, which I personally value more.
Simply because other people disagree.