PostgreSQL uses a write-ahead log (WAL) to write data in a way that survives a database or hardware crash. This is similar to the log buffer or redo log found in other databases. The database documentation covers the motivation and implementation of the WAL at https://www.postgresql.org/docs/current/static/wal-intro.html.
To quote from that introduction:
"WAL's central concept is that changes to data files (where tables and indexes reside) must be written only after those changes have been logged, that is, after log records describing the changes have been flushed to permanent storage."
This procedure ensures that if your application has received commit for a transaction that transaction is on permanent storage, and will not be lost even if there is a crash. This satisfies the durability portion of the atomicity, consistency, isolation...
