What is wallet activity?

Activity is the record of who did what in a wallet — what was created, changed or removed, by which person, and when.

It is not your transactions. Transactions are the money; activity is the actions taken on the records of that money.

It earns its place once a wallet is shared. When two people can both record and edit, "who changed this, and when?" stops being a rhetorical question, and a log answers it without anybody having to remember.