I can explain what it is. I can't explain just why it's drawn precisely that way; that seems to have been a matter of convention.
It's a scale. The vertical lines in it are spaced one scale foot apart. The vertical line at the extreme left is located at the "aft perpendicular," and the one at the extreme right marks the "forward perpendicular." The numbers underneath represent the distances forward from the aft perpendicular.
There are seven horizontal lines, dividing the space between the top and bottom ones into six equal spaces. The horizontal lines, so far as I know, are really only useful at the extreme left; they could just as well be omitted elsewhere on the scale, but rarely if ever are. In the first, foot-long space at the left (stern), the draftsman has drawn an inverted V, with its apex located carefully in the middle of the space. The points where the legs of the V intersect the horizontal lines mark intervals of an inch. If, for instance, you want to measure a distance of three inches, put one point of your dividers at the intersection of the middle horizontal line and the left vertical line, and adjust your dividers till the other point hits the intersection of that same horizontal line with the left leg of the V. That leg of the V hits the second line from the top at five inches, the third at four inches, etc. (Describing it verbally takes at least ten times as long as doing it.)
It's a rather clumsy system; it's obviously easier to use a ruler - and I rather suspect that's what the typical eighteenth-century person using one of these drawings did. (Remember, though, that most Admiralty drawings were originally made on the scale of 1/4"=1', so the V was actually pretty big: 1/4" wide, to be specific.)
Thank goodness they did it that way, though. It's almost impossible for a modern publisher to reproduce such a drawing without including the scale bar. I get explosively irritated whenever I encounter a drawing whose idiotic draftsman has indicated the scale of it simply by writing "Scale: 1/4" = 1' " on it. It's just a matter of time before the drawing gets reproduced in a book, or elsewhere, at a reduced size. (Book designers, who get their training in art schools and frequently have no comprehension whatever of how ship plans work, are notorious for pulling stunts like that.) If the draftsman includes a scale bar, on the other hand, you can always figure out the scale. (It's worth noting that Howard I. Chapelle, in most of his works, used the same, old-fashioned system the Admiralty draftsmen did. Bravo.)
For the modern ship modeler using the plans, all this is of minimal practical relevance. What's essential in working with old drawings like these is to get them reproduced - accurately - to a known scale (preferably that of the model you're building). (Unfortunately that's often easier said then done. Admiralty drafts, being drawn on linen and, in most cases, having been stored rolled up for prolonged periods, are notoriously prone to distortion.) Once you've got a reproduction of the drawing to a known scale, you can use your modern measuring devices and forget the eighteenth-century scale bar.
Admiralty drawings are loaded with lines and other markings that aren't really relevant to most modelers. Through most of the eighteenth century, for instance, the cross-sections of a warship (i.e., the shapes of the frames) were derived from the arcs of circles. Modelers sometimes get baffled by mysterious lines on Admiralty drafts that, in fact, identify the locations of the centers of the circle arcs - which, unless you're building a plank-on-frame model and drawing the individual frames for yourself, don't make any difference to you.
The bottom line is that these things are a joy to study. Anybody who's done any drafting at all has to be in awe of the typical Admiralty drawing - not only for the information in conveys, but for the superb artistic quality of it. Traditional drafting is a dying art; the Admiralty drafts are among the finest examples of that art in existence.
Hope that helps.