Accents are where you play notes or chords a bit louder. It enables the dynamics for a section to stay the same, but highlight certain notes as being a bit louder. They are used by all styles of music, and are a good way to add dynamic rhythm. For example, to make a driving rock groove, you could play 8th notes (1 + 2 + 3 + 4 +), and accent only the numbers. Heavily accented notes can be used for stabs, or ending notes.
Accents are displayed on sheet music as a right arrow > symbol, or as an upward arrow for a heavily accented note.