You may have to take jobs as a dog walker, cat groomer, and fish feeder before you complete school for a career as a veterinarian. Small jobs in the short term give you experience for a long-term career.
Career comes from the French carrière, from an older word for "street" or "road." Staying on track for a certain career is like driving a car in one direction on a road, instead of veering in different directions. Maybe the expression "career path" is helpful in remembering that a career is something that progresses, or moves forward on a set path. It can also be used as a verb to describe something moving quickly and uncontrollably, like a car careering into a ditch.