You stand at the blackboard, daddy, In the picture I have of you,
These two lines are pretty clear-cut. The speaker is looking at a picture of her father, and in the picture he's standing at a blackboard, probably in a classroom, teaching.
An interesting fact: Plath's father was a professor.
A cleft in your chin instead of your foot But no less a devil for that, no not
These lines get a little more complicated.
The speaker says that her father has a cleft in his chin, which still sounds like a pretty normal physical description, but then she says that this cleft is in his chin instead of his foot. What kind of person has a cleft in his foot?
Well, in the next line, we find out that she's not comparing her father to a person – but to the devil. The devil is often depicted as some sort of animal, like a goat, that has hooves and not feet. There's often a signature cleft, or indent, in the devil's feet.
The speaker has moved from calling her father a Nazi, to calling him a devil.
The "no not" at the end of this line is an example of enjambment, a poetic device in which an idea is split between two lines.
Any less the black man who
Bit my pretty red heart in two.
These lines continue the idea that was started in the previous line. Just like the cleft in the wrong place didn't make the speaker's father any less a devil, it didn't make him any less the cruel man who the speaker says bit her heart in two.
The speaker says that her father is a black man, but she's probably not talking about his skin color. Instead, she's referring to him as a dark and evil person.
The color black also contrasts vividly with the red of her heart. Again, he probably didn't actually bite her heart in two. That's just a more vivid, vicious way to say that he broke her heart.
These two lines continue the contrast of the father to the speaker. The father is huge, evil, and black, while the speaker, like her heart, is a pretty, red, and a victim.
I was ten when they buried you. At twenty I tried to die And get back, back, back to you. I thought even the bones would do.
After we're told that the speaker's father, whom she is comparing to the devil, has broken her heart, we're shown a little more how he otherwise affects her.
He died when the speaker was ten years old, and ten years later, when she was twenty, she attempted to die as well.
She says that the reason she attempted suicide was to get back to her father. She repeats the word "back" three times, showing that she's distressed.
We can remember from earlier in this poem that she used to pray to "recover" her father. When she was twenty, she took this further, and tried to die to see if she could be reconnected with him.
She says she thought that "even the bones would do." Maybe she thought when she died she'd be buried near her father, or that once she became only a skeleton, she would be back with him.
Mostly, this line shows how disturbed the speaker is by her relationship with her dead father – so disturbed that she would try to kill herself so that she could come closer to him.