primary structure
Of all the things you could say about a protein, the most basic is simply: what amino acids does it contain, and in what order? That ordered list, read from the N-terminus to the C-terminus, is the protein's primary structure. It is the protein spelled out letter by letter, like writing 'C-A-T' before you picture the animal.
Primary structure is just the linear sequence of residues held together by peptide bonds — nothing about shape yet, only the order. It is written using the one-letter codes, so a fragment might read MVLSPADKT and so on. This sequence is not invented by the protein; it is dictated, residue by residue, by the order of codons in the gene's messenger RNA, which in turn comes from the DNA. So primary structure is the direct, physical handoff point of genetic information into the world of proteins.
Why does the bare order matter so much? Because, as Anfinsen's principle holds, the sequence largely contains the instructions for all the higher levels of folding — change the order and you can change the final shape and function. The clearest example is sickle-cell disease: a single residue swap in hemoglobin (glutamate to valine at one position) makes the protein clump and deforms red blood cells. One letter, out of hundreds, can mean the difference between health and disease.
In sickle-cell hemoglobin, just one of the 146 residues in the beta chain is changed — glutamate becomes valine. That single swap in the primary structure is enough to misshape red blood cells.
A one-letter change in the sequence can change everything downstream.
Primary structure is only the order of residues, not the 3D shape — but it is the order that ultimately encodes the shape. Sequence and structure are distinct ideas that are tightly linked.