Feeling a little overwhelmed. . .:P

So, I just got my "Essentials to Biology" last week and have been reading through it and feeling over whelmed and finding it hard to comprehend.
I took Biology in high school 3 years ago and I loved it then, but am finding that there are soooo many terms in the text book I'm reading now.
I guess maybe it's a silly question, but do I need to memorize and completely comprehend every bold word??? How in depth do I need to understand everything? Is there a good way to find out the essentials of what I'm going to need to know for exams and proctors?
I really desire to do well and be prepared for my exams, but am kinda doubting my ability if I need to know the ins and outs of every word.