When you read, are you meant to think about and pay very close attention to every line?

So, are you meant to try hard to understand the line in every sense? Or are you meant to just breeze through the sentences whilst picking up the general gist of things? That is, if it's a fairly simple book (not like Charles ens). What do you do? From my last question, I have been getting a lot better at reading and comprehension. Thanks :)