When you play notes or chords marcato, you really emphasize them so they stand out sharply. This usually means they are also a bit louder than the notes or chords around them.
Like most musical instructions, marcato is Italian. It literally means "marked" — in other words, standing out from the rest by being strongly accented. There's usually a symbol > or – above the notes or chords to be played marcato. On a piano, you need to use a firm, forceful touch to achieve this effect, and release the pedal before continuing. Other instruments may use different techniques. You can also use marcato as an adjective, as in "Let's hear those marcato chords at the end!"