External validity is whether a study's findings generalize beyond the specific sample, setting, and conditions in which they were originally obtained.
A lot of psychology research, historically, has relied heavily on college students, largely because they're a convenient population for university researchers to recruit. That's fine as far as it goes, but it raises a real question: does a finding about how 20-year-old undergraduates respond to a persuasive message also hold for working parents in their forties, or for people in a different country entirely? If it doesn't, the study might be perfectly sound internally and still have limited external validity — it tells you something true about a narrow slice of people, not people in general.
External validity covers generalization across several dimensions at once: to other people, other places, other time periods, and sometimes other methods of measurement. A finding can generalize well along one of these and poorly along another, which is why researchers are usually cautious about extending conclusions past the specific conditions they actually tested.
It's related to, but broader than, ecological validity, which is specifically about whether a study's setting resembles real-world conditions. There's also a common tension with internal validity: the tightly controlled studies that best isolate cause and effect are often the ones furthest from everyday, real-world messiness, which is one reason replication across different samples and settings matters so much in building a solid body of evidence.