Immeasurably is the adverb cousin of immeasurable: to a degree you cannot measure. You will meet it most with a comparative — immeasurably better, immeasurably worse, immeasurably more complicated — meaning the gap is so big that counting it would be silly. It is an emphasiser, a little literary and a little formal. In speech, 'so much better' or 'far, far worse' does the same job. It does not describe a thing's size by itself as often as the adjective does; it usually tells you how much something has changed or how far two things sit apart.