When you're ready to store/delete data, precede the Set and Kill statements with a TStart command, and place a TCommit command afterwards. This marks the group of statements as a transaction. If the system happens to crash during a transaction, the system (when it returns to operation) will roll back (cancel) the incomplete transaction. If you need to roll back a transaction that has already started (this should be rare), use TRollback, usually with an argument of 1 (trollback 1). Either the entire transaction completes, or none of it does.
After the next exercise, the final few pages of this tutorial will show you how create, update, and delete operations on InterSystems IRIS Objects or SQL Tables are automatically performed inside a transaction.