Throttling is the process of limiting the number of requests  you can submit to a given operation in a given amount of time.

A request can be when you submit the campaigns. Throttling protects the web service from being overwhelmed with requests, messages gets failed with error code 0x00000058 (throttling error, 88 decimal value).

Tips to avoid throttling :
Know the throttling limit of the specific request you are submitting.