What are blocks and how did I reach my limit?

What are blocks and how did I reach my limit?

I am new to Nimbus Note. I imported my notes from EN. I've been using Nimbus for a few weeks now; everything has been going well for the most part, minus the slow loading speeds and what not.

Just today, I was tryign to add to an existing note and I got an alert/pop-up,

"You have reached your blocks limit. Upgrade to premium plan to create unlimited new blocks in your pages."

I have no idea what blocks are. Can anyone shine light on what just happened? I can't seem to access any of my notes in the iOS app. I just tried and was successful in adding to my existing note using the Windows client, so it seems this issue is only happening with the iOS app. (I just checked and there was an update pending; so I updated to the latest iOS version, hoping it would help, but no, the issue is still there)

Thanks in advance!