Definition
Primary stress is the most prominent sounding syllable of a language that assigns stress, with a relatively higher amplitude and pitch. Primary stress is marked with [‘] over the syllable receiving the stress. Words in stress assigning languages will always contain a primary stress.
Secondary stress is relative to primary stress, with a lower amplitude and pitch than the syllable which contains primary stress. Secondary stress is marked with [ˌ]. Stress assigning languages will not always assign a secondary stress.
Examples
Hul’q’umi’num:
INLP Linguistic Glossary 