A silent, high-class ห "leads" low-class nasal stops (ง (ngɔɔ), ญ (yɔɔ), น (nɔɔ) and ม (mɔɔ)) and non-plosives (ว (wɔɔ), ย (yɔɔ), ร (rɔɔ) and ล (lɔɔ)), which have no corresponding high-class phonetic match, into the tone properties of a high-class consonant.
There is one more important thing you have to learn in order to completely master Thai tone in which it's the topic ห- and อ- leads. When you see ห or อ leads an initial consonant, you don't pronounce ห- and อ- sounds but they will change the tone rules of the syllable.