Chunks
A chunk is a contiguous range of data from a particular collection. Chunks are described as a triple of collection, minKey, and maxKey. Thus, the shard key K of a given document assigns that document to the chunk where minKey <= K < maxKey.
Chunks grow to a maximum size, usually 64MB. Once a chunk has reached that approximate size, the chunk splits into two new chunks. When a particular shard has excess data, chunks will then migrate to other shards in the system. The addition of a new shard will also influence the migration of chunks.
When choosing a shard key, keep in mind that these values should be granular enough to ensure an even distribution of data. For instance, in the above example, where we're sharding on name, we have to be careful that we don't have a disproportionate number of users with the same name. In that case, the individual chunk can become too large and find itself unable to split, e.g., where the entire range comprises just a single key.
Thus, if it's possible that a single value within the shard key range might grow exceptionally large, it's best to use a compound shard key instead so that further discrimination of the values will be possible.