A 73 is a C and is one percentage point above a C-.
Aside from 73 out of 100, there are numerous ways students can end up with 73% grades. Here are scales for common number of test questions that can lead to a 73 grade.
If you are scoring in the 73% range on tests, you need to start working a harder if you're thinking about going to college. Check out the discussion about grade averages in this range (2.0 GPA) to see how you're doing, or jump right to the list of colleges you can consider.
Reference table of the percentage grade scale with letter grade and unweighted GPA point equivalents. This grade scale is used my most U.S. middle schools, high schools and colleges.