A chord is a sound composed by more than one note played at the same time.
On bowed instruments, violin, viola, etc., a chord is done by playing two or more string simultaneously, or in more or less rapid succession.
Chords are quite demanding for performers because they require the use of two, three or even four fingers at the same time, with the player controlling the exact intonation of each note and the conduct of the bow on the strings. They require even more coordination than normal playing, the tendency is to press heavily the bow into the strings and the result is a scratchy sound, while instead the right arm should be light and moved horizontally, fast if one wants to play three or four strings together.
In his book "Playing the viola - Conversations with William Primrose", Primrose gives some good avice on how to play the viola, in general, and chords in particular.
Bach's music for strings, especially the Sonatas and Partitas for solo violin, contains a lot of chords, because he often composed polyphonic music (with more voices) and requires the player to make the different voices be heard clearly.