* A period of calmness or stillness: "The storm raged for hours, but then there was a lull in the wind and rain."
* A break in activity or action: "The conversation was lively, but there was a lull when everyone got tired."
* A temporary decline in something: "There was a lull in sales after the holiday season."
Essentially, a lull is a pause or break in whatever was happening before. It can be welcome or unwelcome, depending on the context.