In Junos OS, there are two main configuration databases:

Candidate configuration: a working copy of the configuration that holds any uncommitted changes. All configuration edits are made here.

Active configuration: the running configuration that the system currently uses.

When you issue a commit command, the candidate configuration is promoted to active, replacing the previous configuration. Until that happens, all changes remain in the candidate configuration and do not affect device operation.

From Junos Fundamentals Guide:

“The candidate configuration is a separate copy of the configuration database. All changes are made to the candidate configuration and do not take effect until they are committed.”