Well, it might be the case that you were lacking some assumed prerequisites (don't forget that the book is aimed at MIT undergraduates, who will have some background in math and physics).
But if you cannot relate an exercise to the subject matter you just studied, it might also be the case that you think you understand what you just read, but don't really do. It's easy to read math superficially, honestly believing that one has understood the point, when in fact the "mental image" the authors wished to impart on you has not been grasped.
So you should not ignore the exercises, but work hard at them. They (well, some of them) are supposed to be hard. They are supposed to make you think, not just rehash what you just read. And after all that hard work, hopefully, something will click and you will be ready for the next section.
But if you cannot relate an exercise to the subject matter you just studied, it might also be the case that you think you understand what you just read, but don't really do. It's easy to read math superficially, honestly believing that one has understood the point, when in fact the "mental image" the authors wished to impart on you has not been grasped.
So you should not ignore the exercises, but work hard at them. They (well, some of them) are supposed to be hard. They are supposed to make you think, not just rehash what you just read. And after all that hard work, hopefully, something will click and you will be ready for the next section.
And then you will feel awesome.