Microsoft's Office Insider Program used to only be for Windows users, letting you download beta updates that let you preview experimental upcoming features. Now Apple users can get in on the fun with a preview program of their own.
The Mac Preview Program lets Office 365 subscribers opt into the program by checking the appropriate box from inside the Microsoft Auto Update tool. It's simple and generally the features that get pushed to the insider program don't cause Office in general to be unstable.
The blog post announcing the insider program didn't specify anything Mac users can look forward to, though it's not too much of a stretch to think that whatever features come to the Windows version will also find their way to the Mac version as well.