The witnesses that run steemit will routinely make upgrades to the Steemit system. A lot of times, these require changes to the blockchain itself. That's where the term "hard fork" comes into play. These updates usually result in a new blockchain (still called steem, and keeping all the old transactions through some voodoo that I don't understand).

For you, it doesn't matter too much on the technical level. This particular hardfork (known as HF20) changed the way users on Steemit post, comment, and upvote. In a perfect world, we wouldn't see the change unless we were flooding the blockchain with lots of upvotes, comments, and the like. But this time around, there was a bit of a fluke that caused people who had been around a long time to have no power to do anything. So it was a bit of a shit show trying to get everything stabilized. I think we're out of the woods for the most part, and it shouldn't be an issue anymore.