parent chain
The parent chain is the backbone you pick out of a molecule to base its name on: the longest continuous chain of carbon atoms (and, in later chapters, the one carrying the most important functional group). Everything else hanging off it becomes a side branch. Finding the parent chain is always step one of IUPAC naming — get it wrong and the whole name comes out wrong.
The catch is that the longest chain is not always the one drawn straight across the page. You must trace through every twist and turn, even around corners, to find the truly longest path. A molecule written as a tidy five-carbon row with a branch may actually contain a six-carbon chain if you turn the corner into the branch. The parent chain gives the root name (its length: hex- for six carbons) and the suffix; the leftover carbons are named as substituents on it.
If two different chains tie for longest, you break the tie by choosing the one with more substituents attached (more points of branching), which keeps the descriptive work in the prefixes where it belongs. Picking the correct parent chain is the single most common place beginners slip, because it is tempting to just count the carbons drawn in a horizontal line and stop there.
A structure that looks like '2-ethylpentane' is actually 3-methylhexane: turning the corner into the ethyl branch reveals a six-carbon parent chain, longer than the five you first counted.
The longest chain may run around a corner — always trace, never just read across.
The longest chain is not the one drawn horizontally; it is the longest path you can trace through bonds, corners included. Tie-break by most substituents, then by lowest locants.