This is a motion that needs to be filed when you missed a deadline or failed to do something that you were required to do, resulting in a default and probably a default judgment against you, or some other adverse consequence. Unlike the motion to vacate, this is when it is your fault.
It most typically arises in the beginning of the case, where the landlord's process server falsely claims to have served you with the summons and complaint. When you don't file any response within 5 days of when the process server CLAIMS that he served you, the landlord's lawyer files your default, and requests a default judgment to start the lockout process. Other times, it may happen when you were supposed to file the next paper by a given deadline, but you forgot, or you were late getting to trial, and they went on without you.
The purpose of this motion is to un-do what has happened, to un-lose the case, so that you can continue on to trial, rather than suffer because of some simple oversight.
This $300 does NOT cover the related ex parte stay and shortening time, nor does it cover any related phone calls, emails, faxes, mail, or related events which need to be paid for separately as a consultation. This motion usually requires the ex parte application to stay execution and whatever other paper you would have filed on time.