Could be two things...
1) It could be an air pocket where they failed to adequately vibrate/consolidate the concrete when it was placed. Although not ideal, this is usually not a real big deal unless the voids are significant. Small scale air pockets happen. If it is a few feet long or very deep I'd ask for engineering.
2) It could be a cold joint, or differential pour. If the concrete toward the bottom of the wall is allowed to set up somewhat before the entire height of the wall is placed, it is called a cold joint. It often happens while they wait on concrete or get delayed working on the other side of the house. Cold joints are usually identifiable by slight variations in color, and they're usually somewhat horizontally oriented or somewhat sloped. Whether or not it is a problem depends on a lot of factors. Wet concrete does not bond well to set concrete, so it is a weak point. It it is a cold joint, I would ask that the builder and concrete contractor provide an engineer's report (stamped) either ok'ing or recommending repairs. As an inspector, when I see a cold joint, that's what I require.