What is Isolation level, write-ahead log?

An isolation level states which anomalies of concurrency a database allows between transactions, from read uncommitted to serializable, under which the result is that of some serial order. A write-ahead log records each change before it is applied to the database’s main file, so that a crash can be recovered by replaying or discarding it: in SQLite’s WAL mode "the original content is preserved in the database file and the changes are appended into a separate WAL file".