For they have made ready their heart like an oven, whiles they lie in wait: their baker sleepeth all the night; in the morning it burneth as a flaming fire.

They prepared their hearts like a hot oven while they hide and wait. Their "baker" (a metaphor for their actions) sleeps all night, but in the morning, it becomes fiery hot.

King James Version

Simple English

* Work in progress - there may be occasional inaccuracies