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Scientific validity is the applicability of a conclusion drawn in the context of a scientific experiment to the world at large. Science rarely attempts to answer questions that apply only in a laboratory setting, but when following the scientific method many problems must be reduced to tightly controlled laboratory studies that are far removed from the larger real world questions being asked. An important question then is whether or not the conclusions and data drawn from a tightly controlled study can be extrapolated outside of the lab. The degree to which extrapolation is possible is the scientific validity of an experiment.
 The freshman psychology undergrad problem
An easy way to see this is in one of the classic problems in the field of psychology. In the majority of research universities conducting psychological experiments the primary subject pool are undergraduate students that receive class credit for participating in experiments. This means that there is a very strong systematic bias towards using a particular population in almost all psychological research across the world. If there is something "special" about this particular sub-population that makes them fundamentally different from the population at large then experimental results have no general validity and can only be applied to the sub-population of "college psychology undergraduates."
A similar problem comes in infant studies. A great deal of interesting work is being done on infants, babies and toddlers about mental, linguistic, musical, and sensory development. Much of this research is done using modern imaging devices such as PET scans and EEGs. While these scanners are mostly harmless, they often look foreboding. The population of parents that are aware of infant studies and who are willing to let their babies participate is basically graduate students (which is why universities love it when grad students breed). Again if there is something special about a population of infants whose parents are psychology graduate students then the validity of the experiments to the general population of human infants comes into question.
There are several statistical techniques that help deal with certain validity issues, but mostly they can only be dealt with through good experimental design. The various threats to scientific validity are carefully explored both in the philosophy of science and more directly by researchers themselves.