Construct Validity refers to the ability of a measurement tool (e.g., a survey, test, etc) to actually measure the psychological concept being studied. In other words, does it properly measure what it's supposed to measure? For example, if we want to know our height we would use a tape measure and not a bathroom scale because all height measurements are expressed in inches and not in pounds.
If we want to study and try to measure the abstract concept of love we might use a questionnaire that asks for the person's reactions to certain situations. We will then include some questions where we already know what the answers are based on existing knowledge. When we compare the new scores to the scores of the questions we know the answers to, the scores should be similar. If they are, then the measuring instrument is probably actually measuring the concept of love.