In music, a chord is three or more notes that combine harmoniously. You can play chords on a piano or guitar, but not on an instrument that plays one note at a time, like a trumpet.
Chord comes from the French word for agreement, accord, so in music it means sounds that go together, or agree with each other. You can also use chord figuratively. If a book "strikes a chord" with you, you feel connected to it. In math, a chord is the straight line that connects two points on a curve.