In research a floor effect (aka, Basement Effect) is when measurements of the dependent variable (the variable exposed to the independent variable and then measured) result in very low scores on the measurement scale. This could be hiding a possible effect of the independent variable (the variable being manipulated).
An example could be seen in a study being conducted by a school on the prevalence of academic dishonesty. They ask their students in an anonymous questionnaire how many of them have stolen an exam from a teacher. The results would be very low presuming that most people have never done this in their academic career. The results would suggest that academic dishonesty was very very low in this student population. If they changed the question to be whether a person has looked at another's paper the amount of "Yes" answers would be more prevalent and be more representative of the prevalence of cheating.