A researcher gave a sports drink to 8th-graders and watched them dunk a basketball. If you control for confounding factors, your results are more likely to apply beyond your study. But if your sample doesn't match your target population, the findings fall apart.
For external validity. Do my results apply to the outside world? If I gave the Sport drink to 8th graders and I found that they were able to dunk the basketball, can I translate that to the rest of the world saying, hey, the rest of you basketball players, you should be doing this? Because look what happened to these eighth graders. And if my internal validity has been handled and controlled, then I have a lot better song to sing about translating this out to the outside world. But if again, I've got an 8th grader who's 4 foot 9 and the rest of them are 7 foot 3, I can say 90% of my 8th graders were able to dunk after having this drink. Well, 90% were able to dunk before having the drink too. Controlling internal validity helps your external validity as well. Drop the word help in the comments and I'll send you a link to my free dissertation help community.
