Here's how to break it down:
* Iamb: An iamb is a metrical foot consisting of an unstressed syllable followed by a stressed syllable.
* Tetrameter: "Tetra" means four, so tetrameter means there are four iambs per line.
Let's analyze the line:
Like (unstressed) snow (stressed) cov (unstressed) ered (stressed)
fields (unstressed) on (stressed) a (unstressed) win (stressed)
ter (unstressed) y (stressed) day (unstressed)
You can see the pattern of unstressed/stressed syllables repeating four times, creating the iambic tetrameter.