Money transmission is the activity of accepting currency, funds or other value from one person and transferring it to another person or location by any means. It is mainly a United States legal term, and it matters because doing it triggers registration and licensing duties. The businesses that carry it out, and how the label is applied to apps, wallets and crypto firms, are explained in Money Transmitter.
At federal level, firms offering money transmission services are a type of money services business and must complete FinCEN registration. Almost every state also requires a money transmitter license, under its own money transmission law, before the firm can serve residents of that state. Outside the US the same activity is usually called money remittance or a payment service.