Boost for Reddit was a very popular third-party Reddit app. After Reddit's API changes, causing the third-party apps to shut down, the Boost developer is now making an app for Lemmy.
Gonna take a guess, but it probably was a good client and for many people it was the most enjoyable method to access reddit with it. For me it was Sync, now called "Sync for Lemmy"
Did you ever use the official Reddit app? I personally do not like it very much. The third-party Reddit apps were just a whole lot better, at least in my opinion and others. Apps like Boost and Sync were both very popular Reddit apps, but have both now moved on to Lemmy.
Boost is probably just going to be an Android app, since Boost for Reddit was also Android only. I wasn't sure if the app you were talking about, Memmy, was an IOS app.