Compute your GPA for a single term.
Term GPA is quality points divided by credits, where quality points are each course's grade point multiplied by its credit value. The weighting is the whole point and it is what makes GPA differ from a simple average of your grades. A six-credit course counts twice as much as a three-credit one, so a weaker result in your heaviest course drags the GPA below the arithmetic mean of your grades. Reporting the difference between the weighted GPA and the unweighted mean makes that effect explicit, and it is usually the answer to why a GPA came out lower than expected.
Quality points
Points = sum over courses of grade point x credits
Term GPA
GPA = total quality points / total credits
An illustrative estimate only. Grading scales, GPA weighting and credit rules are set by your institution and vary between programmes and academic years. Confirm all figures with your course handbook, registry or academic adviser before relying on them for any academic decision.
Credit weighting. If your weakest result is in your highest-credit course, that grade counts for more than the others and pulls the GPA below the simple mean of your letter grades. The reverse also holds — a strong result in a heavy course lifts it above the mean.
For GPA purposes, yes. A six-credit course moves your GPA twice as much as a three-credit one for the same grade change, so an hour of study invested there has double the effect on the number. That is worth knowing when time is scarce near the end of a term.