The "N" refers to the nitrogen atom in the "amine group" of an amino acid.
Proteins are built in a specific order. The N terminal residue is always one that was added before any others.
The order of segments is important.
Some protein-viewing tools have "rockets", similar to the cartoon view in Foldit, but the arrowheads on sheets and helixes. The arrowheads point toward the end of the protein, the higher-numbered segments.
The last segment of a protein is called the C terminal.