闪亮 means 'to shine brightly' or 'to glitter'. It is a separable verb (离合词), so the two parts can split when inserting elements like complements or measure words, though the compound form is more common. You can say 闪了一个亮 to describe a quick flash. The dictionary also lists a rare dialectal sense 'dawn', but the core meaning is 'shining, glittering'.