A study has internal validity questions if the experimenter's interpretation of the study is disputable. For example, say a study showed people who drink a lot of tea have fewer health problems and the experimenter said, "This shows tea prevents health problems." The experimenter's statement could be wrong; it might be that tea drinkers on the average have better health habits overall and that factor rather than tea drinking is the real causal factor. So it would be reckless to say tea prevents health problems without considering other relevant factors. Actually in this particular study tea drinkers also (a) smoked less, (b) ate better, and (c) consumed more vitamin supplements than non-tea drinkers. The statistics in this study ended up showing it was the healthier lifestyles rather than tea itself that caused fewer health problems. Since the experimenter's initial statement was debatable, the study has questionable internal validity since there are alternative explanations (plausible rival alternative hypotheses) for the tea drinkers having fewer health problems besides their tea drinking habit. Exactly what it takes to conclude internal validity is established by a study depends on your philosophy of science but, at a minimum, you need to collect data that helps you evaluate how important alternative explanations are in explaining the dependent variable you are interested in. (An optional advanced tutorial on internal validity threats based on the book by Donald T. Campbell and Julian Stanley, Experimental and Quasi-experimental Designs for Research, is available at http://psych.athabascau.ca/html/Validity/ This goes into more detail than is required for Psyc 103 but is interesting for the curious student.)
Of course, science is not made by one study, replication is the essence of science, so you would need to look at a number of studies before you could conclude tea is healthy or does not really affect health. If you do this you would discover there are a number of different studies using humans, animals, along with test tube studies of the chemicals in tea, and the overall conclusion is that tea does seem to actually have health promoting effects. So while the first study discussed had an internal validity question, the overall evidence supports the conclusion tea does have healthy effects.