What type of metrical foot consists unstressed syllables directly followed by a stressed syllable?

The metrical foot that consists of an unstressed syllable directly followed by a stressed syllable is called an iamb.

Here's an example:

* be | lieve (The word "believe" has the stress on the second syllable)

Other common metrical feet include:

* Trochee: Stressed syllable followed by an unstressed syllable (e.g., "HAP | py")

* Spondee: Two stressed syllables (e.g., "NIGHT | MARE")

* Dactyl: Stressed syllable followed by two unstressed syllables (e.g., "FLA | min | go")

* Anapest: Two unstressed syllables followed by a stressed syllable (e.g., "un | der | STAND")