When the waves that come together have the same frequency they are said to be in phase. Waves that are in phase add up and result in one bigger wave in terms of amplitude. For ocean waves this means larger or higher waves while in optics this means a brighter light. The adding up of two waves is called constructive interference.
On the other hand, when two waves that are of different frequencies meet they are said to be out of phase. Since they are not in phase the waves, instead of adding up, cancel each other out resulting in a smaller wave. This kind of interference is called destructive interference.