Documents = how many things are searchable. So this could be posts, database records, or files, or literally anything that shows up as a search option.
Shards = How many "instances" of the index is created. Each instance can only handle a certain number of queries and requests at one time. To help scale super large data sets with lots and lots of searching occurring, multiple shards are deployed to help handle lots of activity at once. In most cases, you don't have to worry about this.
Read/Update (or sometimes called Write) is the amount of activity associated with how often you pull info from the index or put data back into the index. This is not something that many providers go to the level of spelling out. I would personally not worry about it too much unless you have a huge board that has TONS of people searching or that you have Similar Content widget showing on every page that has to query the index to show relevant topics for each topic view.
Ultimately the biggest limits you need to worry about is the total disk size and how many records you can have. That should handle for a "majority" of people what they need. (There are obviously situations where that is wrong, but those tend to be more corner cases than the common use case.)