Dramatic irony is when the audience has more information about a situation than the characters in a fictional work do. Such irony is expressed when a character's words or actions have a significance that the audience comprehends but the characters themselves do not.
Thornton Wilder's Our Town is suffused in dramatic irony since the audience is given far more information than the characters are regarding their fates. In the first act alone (set in 1901), the stage manager informs the audience that Dr. Gibbs and Mrs. Gibbs will soon be dead and that the promising young Joe will perish in World War One. All of this is hard-hitting for the audience, not only because the characters are everyday people they might recognize in their own lives, but because it is much harder to see the characters going about their lives and cherishing particular hopes, knowing they will never come to pass.
There is another ironic scene in the play, but it works on a more metafictional level: when the deceased Emily relives her twelfth birthday party. This time, a character within the play (in this case, Emily) takes on the role of the audience, watching her past self. The irony is that she now understands just how precious life is, while her past, living self never did.