That activity type simply means that they were sent the email.
Just saw a similar post (Activity Type: Send Email?)
"Send Email" is the first activity that occurs during the "Send Email" flow step. Eventually, the second step is "Email Delivered" or "Email Bounced/Soft" and the third activity is "Open Email". So if the campaign ran for 100 leads you will likely see 100 "Send Email" activities and hopefully 100 "Email Delivered" activities.