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A 'sonority hierarchy' or 'sonority scale' is a ranking of speech sounds (or phones) by how much 'sound' they produce. For example, if you say the vowel [a], you will produce much more sound than if you say the plosive [t]. Sonority hierarchies are especially important when analyzing syllable structure; rules about what segments may appear in onsets or codas together, such as SSP, are formulated in terms of the difference of their sonority values. Some languages also have assimilation rules based on sonority hierarchy, for example, the Finnish potential mood (e.g. ''-tne- → -nne-'').
Sonority hierarchies vary somewhat in which sounds are grouped together. The one below is fairly typical (1 indicates lowest sonority):

#	Type	Son/Obs	Cons/Vow
1	plosives	Obstruents	Consonants
2	fricatives
3	nasals	Sonorants
4	liquids
5	high vowels		Vowels
6	non-high vowels