To hold back means to lighten something on the image, i.e. to dodge. The term is really in reference to what you are doing with the light from the enlarger, not to tonality directly. In other words, you are holding back ("blocking," more literally) light from the enlarger from reaching the paper. For example, "I think that your over all exposure and the filter you are using are perfect; you just need to hold back the shadows a bit on your next try."

For highlights, you might say "pull down," "rein in," "burn in," "darken," etc.