In phonology, a heavy or strong syllable is considered to be with a branching rhyme, i.e. one that has a long vowel or diphthong nucleus or a vowel followed by a coda. The typical patterns are VV, CVV, CV+C, CVCC, CVVCC & CVVC. On the other hand, a weak or light syllable is one with no branching rhyme, i.e. one that has a short vowel with or without a consonant following. The typical patterns are V, CV & CVC. For example, ‘rain’ (/rewn/) and ‘see’ (/si+/) are heavy syllable; the 1st syllable of ‘reduce’ (/rw.0dju+s/) and the 2nd syllable of ‘father’ (/0fY+.ðcr/) are weak ones.
As heidita indicated, a heavy syllable is usually the one that gets stressed in a multi-syllable word, never a weak one. Some heavy syllable is unstressed in a multi-syllable word. For example, in ‘dialect’, the 3rd syllable is not stressed.