A milestone is whatever you want to make it--many people make milestones for seed germination, bloom times, etc but you could make them for other things if you wanted. Once you've created a name for a milestone ("started blooming" for example), then you can go into any of the plants in your journal and add a "started blooming" milestone on the date that plant started to bloom.
friend, thought I'd share how I've done it. I use it, as ecrane3 said, to mark "blooming begun". I also added milestones for "cut back". The only problem with making it a milestone instead of just a regular entry is that you can't add a pic. The good thing is that the milestones appear together and you can see the first bloom date for the plant for several years.
I've begun using my journal heavily. I suggest you look at several people's journals to get an idea of how you'd best like to use this great tool.