I've had a lot of questions lately from students about how
important transcripts are in the graduate admissions process. Your application
to graduate school is one of the few times anyone will actually read your
undergraduate transcript so it has some importance. However, how much it
matters depends on a variety of factors.
First, as I have explained before admissions committees (mostly composed of faculty and graduate students) look at several items: letters of intent (also called a statement of purpose), planning-relevant experience, letters of reference, GREs, grades, and (sometimes) work samples. Thus grades are only part of the picture.
In addition, there is great variation among universities and even majors in GPAs and in grading. Many international universities use very different scales. Some grade substantially lower than the U.S. and others are the reverse. Even within the U.S. there is great variety in cultures of grading and recent grade inflation means that it is even difficult to compare people from the same institution 10 years apart. Some universities don't provide grades but rather narrative evaluations of students. That means that while your GPA and grades matter some, admissions committees place them in context.
When reading transcripts, most admissions committees will focus particularly on how well you did in planning-relevant classes. If you received a D in yoga and a C in advanced biomedical robotics it is less important than receiving the same grade in introductory statistics or a class on professional writing.
On the other hand I have noticed some students trying to take easier classes so that they can get high grades. My own institution recently stopped publishing median class grades for courses in order to reduce this practice some, but word still gets around. Certainly you don't want truly terrible grades. However, admissions committees do look at how difficult the classes appear to be and read letters of reference and transcripts carefully to see how you did relative to your peers.
If you received a number of Cs and Ds (or their equivalent), however, you will have to do a fair bit of work to show you can manage graduate school. Perhaps you were really ill, had a major family crisis, or like many others had difficult first year in college. However, often such grades indicate you didn't treat academic work very seriously, particularly if you received them later in your college career. Years of responsible and high-quality output in the work place, good GRE scores, and compelling letters from current employers and former faculty who can attest to how you have turned things around will help a lot.
If your transcript indicates you withdrew from a class or two each year that isn't terrible. In some universities one has such an indicator if you drop a class the first few weeks in the course "shopping"period-in others it's only if one withdraws late in the semester. Admissions committees won't know. However if there are many incompletes and withdrawals it may raise a red flag that you will need to have addressed in either letters of reference or your statement of purpose.
For those wanting to get into undergraduate programs in planning and related fields, the story is fairly similar with high school transcripts. They matter but so do test scores, letters of reference, statements or purpose, and your experience and activities. If you had a bad year it helps if you can show you improved the next year.
I have also provided advice on Getting into graduate school in planning: how to decide if planning is for you, whether to get work experience before you go, find the right program, apply, write a statement of purpose, obtain letters of reference, visit successfully, and decide which offer to take up. Also, whether to do a PhD or not.