Depending on how you look at this term it can be used to define the base skin tone which would then help in the selection of proper hair colors or it can be used to definine different hues of hair color.
Warm tones tend to be based around golds, yellows and brown undertones. Warm tones can be soft or well defined. Other example of warm shades would include various shades of red such as tomato or brick. It would include shades of sunny yellow, coffee, mocha and bronze colors. White blonde or rich carmel or deep taupe would be great warm hair colors or could be descriptions used to define warm skin tones.