There will we sit upon the rocks,
And see the shepherds feed their flocks,
By shallow rivers to whose falls
Melodious birds sing madrigals.
There I will make thee beds of roses
And a thousand fragrant posies,
A cap of flowers, and a kirtle
Embroider'd all with leaves of myrtle;
A gown made of the finest wool
Which from our pretty lambs we pull;
Fair linèd slippers for the cold,
With buckles of the purest gold;
A belt of straw and ivy buds,
With coral clasps and amber studs;
And if these pleasures may thee move,
Come live with me, and be my love.
Let’s just pass over the faintly ridiculous imagery in this line in silence (a belt of straw?!) and focus on how it shows the relation of stress to meaning: 
 A  belt  of  straw 
 This phrase, two perfect, clean iambs, has all its stress on the semantic words– i.e., the words which carry the meaning. The other words (‘A’ is called an article, and ‘of’ is a preposition) essentially frame these words. You can sort of tell what’s going on with ‘belt straw’, ‘A of’ means precisely nothing. 
 SO 
 A key factor in deciding stress is the weight of meaning of the word. But it’s not the only factor, by a long way.
The Passionate Shepherd to His Love