The N terminal of a protein is the first segment (or residue) in the protein.
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.
"Rockets" cartoon view of protein in Jmol. The arrowheads on the helixes and sheets point toward the C terminal of the protein, and away from the N terminal.
Methionine is always the first amino acid added to a protein, but it's often removed by a post-translational modification (PTM).
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.