This is kind of a Big Deal for Mac users who are using older Macs running OS X 10.4 (Tiger) and 10.5 (Leopard), particularly those who are still using PowerPC-based Macs.
As of May 18, 2015, Dropbox will end support for both OS X 10.4 and 10.5. What this means is that users of those OS X versions will not be able to use the desktop application on those Macs.
NOTE: You will still be able to use the Dropbox website to upload/download files, and will still be able to use your iPhone/etc. clients, and other compatible devices/computers.
Here is the official announcement and there is a very active discussion thread in the Dropbox forums.