The number records how many times two or more threads are trying to update the same document inside the WiredTiger storage engine. It is not persisted between server restarts.
This statistic is specific to the WiredTiger storage engine (as it is under the WiredTiger section). Under MongoDB this is known as write conflict, and will be transparently retried under valid conditions.
That is, the update is retried by MongoDB (using the WiredTiger's API) until it completes without a conflict. From the client perspective, the write succeeds and is otherwise normal except for increased latency.
Write conflicts indicates that multiple clients are trying to update exactly the same document within the same fraction of a second. This window is the time between the execution of the WiredTiger API's
WT_SESSION::commit_transaction() operations. There is no fixed minimum time for these operations. It could be double or even single-digit microseconds if the amount of data to be written is small and the server has no queues for CPU or memory channels. There is no fixed maximum time either, although it is somewhat dependent on the performance of the server.
These conflicts can arise due to WiredTiger's optimistic concurrency control, which is described in concurrency FAQ for WiredTiger.
Note that comments in WiredTiger's source code might be misleading from MongoDB's point of view, since WiredTiger and MongoDB uses mostly the same term, but attach slightly different meanings to them.
For example, "transaction" in WiredTiger has always been available, but "transaction" in MongoDB means a multi-document ACID transaction feature for MongoDB 4.0 (not yet released as of this writing).