A qualification is either a condition that must be met or a statement that puts a limit on a claim. Both kinds of qualification are restrictive.
You know how there are requirements for jobs? You can call those requirements qualifications. For example, a college degree and certain experiences are qualifications for many jobs: without them, you won't even get a job interview. A qualification is also a type of statement that makes a previous statement more specific. If you said "I'm thirsty!" and then added "Not for root beer, though," your second statement is a qualification. Qualifications add a limit to what's been said.