An accent is playing a particular note more strongly and more prominently than the others. It is not only about raising the volume: you can also articulate it more firmly, hold it longer, or brighten its tone. Where you place accents changes the rhythm and character of the same phrase.
Strong beats and accents
In 4/4 time, beats one and three tend to feel strong, and these are called strong beats. You can place accents on these natural strong beats, or deliberately on weak positions or offbeats. The latter leads into the feel of syncopation.
Adding them when programming
In MIDI, raising velocity makes a note sound stronger. Depending on the instrument, though, it changes not just volume but timbre and attack as well. Raising only the strong notes slightly, to create contrast with the rest, sounds natural.
Accents beyond volume
You can create accents without volume too: layering a crash on drums, cutting a chord short, leaping to a high note in a melody. When thinking about dynamics, it helps to separate the large arc across the whole song from the small accents within a phrase.
Effect on groove
Even with identical timing, moving where the accents fall changes the groove. Try small changes first, such as strengthening only some of the hi-hats or pushing certain bass notes forward.
Use them with intent
If every note is strong, nothing stands out as important. The basic approach is to decide which rhythm, word, or melodic peak you want heard, and put the accent there.
