The purposes of this paper were to present what understanding of reading was and to show how two major abilities of reading comprehension, summarization and inference-making, affected reading comprehension. Three levels of understanding, including surface code, textbase, and situation model, were discussed. Studies of summarization and inference were reviewed for explained how readers understood the text. In addition, the difference between comprehension and learning was also discussed. Several reading strategies and an instruction model were proposed for educators to facilitate comprehension and learning.