Stanza I Summary

Get out the microscope, because we’re going through this poem line-by-line.

I felt a Funeral, in my Brain,
And Mourners to and fro

The speaker feels a funeral inside her brain.
The funeral is just starting, and the people who have come to mourn the dead person are milling around aimlessly, as people tend to do at tense social gatherings.
It goes without saying that you can't actually host funerals in your brain. We're in the world of symbolism here.
There are two other interesting things about this opening. First, the speaker doesn't want us to think she is comparing some other feeling to a funeral. She doesn't say, "It felt like a funeral in my brain." No, she says, "I felt a Funeral." In other words, the funeral seems totally real and not just the product of an overactive imagination.
Second, she uses the anatomical word, "brain." The funeral starts out as a physical sensation. We'll see where she goes with this.

Kept treading–treading–till it seemed
That Sense was breaking through–

The mourners keep walking inside the speaker's brain until it feels like sense is breaking through. That is, until she begins to experience the funeral as a physical sensation – as something she can perceive with her five senses.
She repeats the word "treading" as if she's not happy about all this pacing back and forth.
Also, "tread" suggests that the mourners are somehow violating her space. You may have heard the famous motto from the American Revolutionary War: "Don't tread on me." Well, the mourners are treading on the speaker.
Finally, watch for images of cracking or breaking in this poem.