In some videos and questions there is the term shard or sharding but I can’t really find out what it means ?
Roughly speaking, it means the data is scattered to chunks across multiple mongod processes.
See more accurate definition here: https://en.wikipedia.org/wiki/Shard_(database_architecture)
Watch this video: https://www.youtube.com/watch?v=39-RLo4B9-Q&ab_channel=MongoDBUniversity
You can study more about it in chapter 3 of this course: https://university.mongodb.com/mercury/M103/2021_April_13/overview
Or read the documentation here: https://docs.mongodb.com/manual/sharding/
Please refer to the updated course link for M103: MongoDB Basic Cluster Administration.