In the past ten minutes I've seen wohl used as: probably, certainly, must have and well. Thoroughly confused, I tracked down this article: http://yourdailygerman.wordpress.com/2013/07/02/meaning-wohl/.
It's a really long article, but it's very thorough. However, what I got from it was it either means 'well' in some cases, or it assumes things, like in probably or must have.
It can also mean 'yes', but I haven't seen it appear on Duolingo like that yet.