Dissection is the process of separating something into pieces. Whether the dissection involves taking a poem apart line-by-line to learn its meaning or cutting open a frog to study its insides, in both cases you're pulling out the parts that make up a whole to better understand it.
Let's dissect, or take apart, this word for a moment. Dis- means "apart" and section means "to cut", which come together to form the definition of dissection: "to cut apart." When you cut something up into sections you're doing a dissection. The word is commonly used when talking about the process of cutting something apart for scientific purposes, it's also used to talk about taking apart an argument, a poem, or a plan.