"Read the syllabus" involves much more than just glancing through in August / January, and you'll find that there are significant benefits to having the syllabus available throughout the course of the semester. For most professors, the syllabus functions as both a class calendar, and as a kind of contract. The syllabus contains most of the information that you will need to be successful for the class. Every professor writes their syllabus a little different, but almost all syllabi share some common components. Here's what to look for.
Grading policy: Each syllabus will list the course grading policy. This includes how many points various assignments are worth and how many points or what percentage is required to achieve an A/B/C/F. Knowing the class grading policy will allow you to calculate your own grade in the class and keep track of your progress over the course of the semester.
Class Policies: This section will include everything from attendance policy, extra credit policy, and late work policy, to policies about children in the classroom. Looking over these policies at the beginning of the semester will give you a good idea of what the professor is expecting from you. Knowing these policies will also save you from emails like "can I get extra credit for doing xyz," when your professor has a "no extra credit" policy. Knowing these policies will also give you an idea where you have some wiggle room in the course. Does the professor require that you complete just 8 out of 10 writing assignments to get full credit? Do they allow a certain number of absences before they begin penalizing your grade? In other words - this is the contract portion of the syllabus. The professor is laying out the reciprocal guidelines of behavior in the course regarding what the professor and students can expect from each other.
Assignment Requirements: This section will give you a brief overview of what kinds of assignments you'll be facing over the course of the semester. Here's where you can (likely) find the format of exams, the basic description of any papers you might be expected to write, homework assignments that will be required, and so forth. Again, this section will contain important information to help you be successful in individual assignments. For instance, online classes may have discussion boards that require that you make a post as well as respond to a classmate - guidelines for what the professor will be grading on in your post and your response will likely be set forth in this section of the syllabus.
University Policies: This section is likely going to be boiler plate across all your classes. Most universities have a set of university policies that they require professors to include in the syllabus. There is important information here. This includes information about plagiarism and academic honest (and consequences), you'll also find information about disability accommodations, safety, and other university-wide policies. You might not need to read the same boiler plate information for 3 different classes, but you should read it at least once.
Course Calendar: This section of the syllabus is typically the last. It will give you assignment due dates, required readings for each week, days when class is scheduled not to meet, and exam dates. You should enter major deadlines in your calendar right away so that they do not sneak up on you. One thing to note: for most college syllabus, readings should be completed before the class date for which they are assigned. So, if your syllabus says "Tues Aug 10: Read Chapter 2," this indicates that you should have read chapter 2 in advance of the class on August 10th.
Once you have read through the syllabus, make sure you have saved it somewhere you can refer back to it. Consider printing the class calendar and taping it on the wall above your desk. And before you email your professor about late work, attendance, extra credit, or your grade - check the syllabus. You might still decide to email - that's fine, but your professor will greatly appreciate it if you indicate that you know the policy and need to ask for some kind of flexibility or additional detail, instead of emailing them to ask a question to which you should already know the answer. The syllabus is the equivalent of the class "bible." Use it!