"Throttle" is an email limit located in the Settings tab of any given sequence. The throttle setting asks one basic question:
"How many prospects can go through step 1 per day?"
"How many emails do I want to go out on step 1 per day?"
This can help to stagger the volume of prospects moving through a sequence at one time, protect you from hitting your email provider's mailing limits, and can also help users iterate more quickly when A/B testing.
If a user sets the throttle limit to 50, that user can then add 200 prospects into the sequence and ensure that only 50 will actually send every 24 hours.
The remaining 150 prospects that have not been emailed will stay in a "Pending" state until the next 50 are activated 24 hours later.
Another benefit to throttling is follow up management. In addition to controlling the amount of email's that go out each day, you want to make sure that you give yourself breathing room to follow up.
If you send out 300 emails a day, even a small fraction of those emails having responses is a lot of follow up. Throttling will make sure that you're able to have breathing room to give each person who responds an appropriate follow up.