If something's pent-up, it's restricted or held back in some way. You might appear calm and collected, but if you've secretly got a lot of pent-up anger, it will eventually need to be released. Boom!
Use the adjective pent-up when you're talking about repressed emotions or stifled feelings or impulses. You and your friend might attempt to remain solemn through a pompous speaker's terrible lecture, only to burst out with pent-up laughter as soon as the talk is over. Many people keep negative feelings (like sadness, anger, or frustration) pent-up when they're not comfortable revealing them.