The SMTP protocol is used to send email. When you send an email the protocol that is used to get it to where you were sending it is SMTP.
SMTP is the set of rules and commands used by various computers and devices to know how to send emails. SMTP is used only to send emails. POP and IMAP are protocols used to get and work with emails.
SMTP is an acronym for Simple Mail Transfer Protocol. It is the protocol that has been used for many years to send emails. Usage of SMTP predates the introduction of the web in the early 1990s. It was used on the Internet prior to that and on other network systems.
You may see SMTP if you need to set up a program that will be sending emails. Besides setting up programs on your computer that will need SMTP to send email, other devices may be used to send email also. I worked on a copier today needing to change its email settings to send email. If you scan items ion that copier you can choose to have the files sent to your email. Other copiers just save the ability to save the scans to a flash drive. In this case the email address that it would be sending from was needed to be changed in the machine so I have to change the name of their email provider and the SMTP protocol to where the protocol is now so it will use the correct one.
SMTP is an important protocol everyone uses all the time when they send email, but usually is in the background and most (actually almost all) people do not know they are using it.