Caesura is a pause in the middle of a line of poetry. It was originally meant as a place for Anglo-Saxon scops to draw in a breath given the length of the poetry they were reciting. While the use of the caesura is still used for this reason, other may use it so as to provide a place where a reader can reflect on what is being said.
In regards to Frost's poem "Out, Out--", the use of the caesura provides a reflection, or pausing, point so that the reader can reflect upon the action, or message, in the poem. In the following lines (7-10), a caesura is used:
And the saw snarled and rattled, snarled and rattled,/ As it ran light, or had to bear a load./ And nothing happened: day was all but done.
The caesura takes place where the commas and colon exist in the lines. Frost is wanting the reader to pause to consider the seriousness of what is going on. The repetition of the "snarled and rattled" represents the seriousness of the threat. The second line is allowing the reader to think about the circumstances. The last line functions as a conclusion to the immediate action. The last line also repeats the thought of the end being there; just in a different way each time.