Well there are no further details out yet about the zerg units so the closest we can get is by trying to interpret the text.

I'm guessing that it's one of two things:
1. the broodlings are spawn when a unit is hit by the guardian and the unit dies (allthough this raises other questions like does the amount of damage done by the guardian to the unit matter or only the final stroke?)
2. it's a replica of the (ex)queen's ability "spawn brood" which seems more likely to me.

I for one am really curious about the new queen unit, about it's role in managing the hive and how it would stand against let's say a thorr.