Translation:People are boring.
Even if people are boring, what truly matters is how happy they are. You may not have crazy dreams like: being an astronaut, winning an Olympic medal or breaking a world record, but you may have a peaceful life filled with people you love. Nothing matters unless you are having fun.
In most cases, it's through context. If you mention that they were at a movie then say "사람들이 지루해요.", it's pretty clear that you mean they're bored.
And if you were at a presentation and said, in reference to the speaker, "사람이 지루해요.", it's clear that you mean that person is boring.