instagram new feature

Hey guys, I saw this new feature "add alt text" when you wanna post a new pic and from the info that insta gave me I couldn't find the difference between this and the main caption of the pic. can you put keywords (hashtags) in description? what exactly it does?

I just saw an article posted about how Instagram is rolling out a new feature for our feeds. I was curious what you all thought of it. I don't imagine this really affects blackhatters but for those who follow all those Instagram guides, it might do damage http://r29.co/2flPMIY