Okay, so you know how sometimes people who do really good things are remembered and celebrated? Well, in the 12th century (that's a fancy way of saying a long, long time ago), there were a lot of people who did really good things and were remembered as saints or blesseds.
A saint is someone who is recognized as holy and is believed to be in heaven with God. A blessed is someone who is on their way to becoming a saint, but they haven't quite gotten there yet.
Now, let's talk about the chronological list. That just means that the list is in order from the earliest time period to the latest time period. So, the list of saints and blesseds in the 12th century is just a big list of all the really good people who were recognized during that time period, and it's organized from oldest to newest.
There were a lot of saints and blesseds in the 12th century, and I don't think you want to hear me list them all out loud, but some of the famous ones were St. Dominic, St. Francis of Assisi, and St. Hildegard of Bingen. These people did really good things, like preaching about God, helping the poor, and writing beautiful music.
So, in summary, the chronological list of saints and blesseds in the 12th century is just a list of all the really good people who were remembered during that time period, and it's organized from the earliest time period to the latest time period. These people were recognized for doing really good things, and some of them are still celebrated today.