If you see Throttled next to a status for any step in a campaign for a contact it means 1 of the following:

1 - You have exceeded your account level setting for the number of emails to send per day... the default is 250.  Edit your preferences

2 - You tried to send 2 automatic emails to the same person within an 12 hour period
​--- We try to protect you from common mistakes in sequences and make sure you don't send too many emails too fast to the same person.

3 - You paused a campaign during the day and there were emails queued to send.
​ --- When you resume the campaign these will be sent first on the next run

4 - Our system detected that we were going to send the email after your Email Cutoff Time setting for the campaign
​--- We will run these first on the next campaign run