The speaker in this poem is alone, somewhere outside in a wooded area on a stormy, windy, snowy night, but for some reason the speaker is unable or unwilling to leave. "A tyrant spell" is holding him/her and preventing his/her departure. As the night grows darker and the winds get colder, the speaker "cannot, cannot go" - repetition that lends emphasis to the determination to remain in spite of the conditions. Cold, dark, night - all are frequently symbols of death; the speaker seems to be in a perilous place, but something is holding onto him/her and not allowing death to come and take him/her.
In the second stanza, "the storm is fast descending" and is affecting things, and perhaps people, around the speaker, as "giant trees are bending, their bare boughs weighed with snow." Still, the speaker "cannot go."
In the third stanza, the speaker has visions of "Clouds beyond clouds above me" - heaven, perhaps - and "Wastes beyond wastes below" - the depths of hell. There is a power or wish to live that is stronger for the speaker than these powerful images, however.
"But nothing drear can move me, I will not, cannot go." The addition of "I will not" emphasizes the speaker's determination to remain, not allowing all the perils and troubles to conquer his/her spirit.