How can you understand poetry?

Here is the answer by Keith Murray from the band We Are Scientist.
Ah, but that is the point of poetry: to narrowly skirt the line on whose other side lies total gibberish. A poem should suggest meaning, but you should never be sure of what it's saying. A poem that you fully understand is a terrible poem that fails in poetry's one objective: to mystify the reader.

Here is an example of a perfectly good poem:
I raked leaves today
down
down
down
down
down
off the roof onto
your job interview came in the mail?

Powerfully suggestive, endlessly evocative, but ultimately impossible to parse.

Here, by contrast, is a shitty poem:
Mark lent me his ruler this morning

What makes it bad? The lack of mystery, of the suspense of un-knowing, or antiknowing, or "knowing without knowledge".

Let's take a look at another great classic that you'll probably recognize from school:
Running my hand through your hair
my finger caught a knot.
I pushed gently
but the knot wouldn't not not not not yield.

Did the knot come untangled, did it "yield"? Did the speaker get his hand out of the other person's hair, or is it still stuck in there on that knot? The poem leaves this question open, with the possibility of several alternatives. "Not" is repeated so many times that the reader would have a tremendously difficult time figuring out exactly how "yield" is being is modified. The puzzle is rendered utterly insoluble by the addition of the homophone "knot", which multiplies our confusion to an irreducible degree.

Is that guy's hand stuck in that girl's hair? Great fucking poem.