Like an iamb and a trochee, a spondee, too, is a type of foot in a metrical feet. In a spondee one stressed syllable follows another stressed syllable. To write a poem using only stressed syllables would be quite a task; so, spondees, typically, are used by poets as an irregular foot in a metre and are referred to as an irregular feet.
Cry, cry! Troy burns, or else let Helen go.
Cry cry | Troy burns | or else | let Hel | en go
In this example, from Shakespeare's Troilus and Cressida, there are two spondees one after the other and this is called a dispondee. When poets break a metrical pattern it is with a purpose; the break informs the audience (in the case of a play) and/or the reader of a shift in emotion or mood in the play or poem.
In Romeo and Juliet Lady Capulet says:
Fie, fie! What are you mad?
Fie fie | What are | you mad
The word fie is an exclamation, and in this case by having the word repeated as a spondee, Shakespeare breaks his usual metrical pattern by being emphatic about the situation to inform and engage with his audience.