The explanation is as follows:

The transaction command is used to find transactions based on events that meet various constraints12.

Transactions are made up of the raw text (the _raw field) of each member, the time and date fields of the earliest member, as well as the union of all other fields of each member1.

The startswith option is used to define the first event in a transaction by specifying a search term or an expression that matches the event13.

For example, | transaction clientip JSESSIONID startswith="view" will create transactions based on the clientip and JSESSIONID fields, and the first event in each transaction will contain the term “view” in the _raw field2.