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