A checkpoint in WiredTiger is basically a snapshot of the state of the database where all data files are consistent with each other.
WiredTiger implements a write ahead log in the form of a journal. That is, a write that is written to the journal is considered durable (i.e. will survive restarts).
See WiredTiger journal and WiredTiger Storage Engine for a high level description of how the process works.
The journal files are only used for recovery purposes in case of unclean shutdown. During normal operation, once the dirty pages in the cache (i.e. in memory) are checkpointed and marked clean, WiredTiger will clean up the now-unneeded journal entries.
Please note that these are quite specific implementation details, and may change between MongoDB versions. Out of curiosity, what is the reason for the question? Are you simply curious about how things work under the hood, or is there another reason?