The phrase is often used metaphorically to indicate a deviation from truth or accuracy. You're off beam if you think I was at the party; I was home all night.
Pro Tip 2/3
Negative Connotation
Implies a mistake or error. Use it to gently point out an incorrect assumption or understanding. I feel his analysis is off beam in assessing the market trends.
Pro Tip 3/3
Conversation Correction
Helpful for correcting a person during a discussion without being overly harsh. I think you're a bit off beam on that issue.