Whose woods these are
I think I know.
His house is in
the village though;
He will not see
me stopping here
To watch his woods
fill up with snow.
The repeated pattern here: [unstressed syllable-stress syllable] or an Iambic foot
Now count to see how many feet there are per line:
[Whose woods] [these are]
[I think] [I know].
[His house][ is in]
[the vil][lage though];
[He will] [not see]
[me stop][ping here]
[To watch] [his woods]
[fill up] [with snow].
There are FOUR iambs per line, so we call that tetrameter.
Hence, each line of this stanza of the poem is in "iambic tetrameter."