Usually, learners of English study at school two-level analysis of stress which means there is a simple distinction between syllables. Syllables are classified as stressed and unstressed with no intermediate level. However, we usually have to recognize at least one more intermediate level of stress.
In order to exemplify the levels of stress, I chose the word "around" which is used in many books to exemplify this.
The word "around" is made up of two syllables. The first one is /ə/ and it is very weak. The second syllable is /ˈraʊnd/ and is a stressed syllable. What is interesting about the stressed syllable /ˈraʊnd/ is the fact that the tone of the voice does not remain constant or leveled. It usually falls from a higher to a lower level as is shown below.
The two parallel lines represent the higher and the lower pitch of the speaker. The prominence in this tone movement gives the strongest type of stress. It is called "primary stress".
Another interesting word that I hear every day and got my attention is "photographic" /ˌfoʊ təˈgræf ɪk/. The first syllable is unstressed and the second syllable is stressed. However, the first syllable sounds a little bit like having a kind of stress that is weaker than the primary stress but stronger than the first syllable of the word "around". If you look at the phonemic transcription of the word "photographic" /ˌfoʊ təˈgræf ɪk/ (exactly as I copied from dictionary.com website) you can see that it starts with a low mark (something like a comma) which represents the stress I mentioned above. It is called "secondary stress".
So, we can say that we have three levels of stress: primary stress, secondary stress, and unstressed. It is worth mentioning that unstressed syllables containing /ə/, /ɪ/, /ʊ/ or a syllabic consonant will sound less prominent than an unstressed syllable containing other vowels. This can be used to identify other levels of stress but this seems to be too complex and unnecessary.