Every pic description is a little program describing drawing actions. The [gtn]roff-dependent versions compile the program by pic(1) into gtroff(1) macros; the pic2plot(1) implementation uses a plotting library to draw the picture directly. Programs that process or display gtroff(1) output need not know or care that parts of the image began life as pic descriptions.
The pic(1) program tries to translate anything between .PS and .PE markers, and passes through everything else. The normal definitions of .PS and .PE in the ms macro package and elsewhere have also the side-effect of centering the pic output on the page.
If you make a mistake in pic input such that the program cannot interpret what you mean, gpic(1) issues a diagnostic message in the format prescribed by the GNU Coding Standards. A typical error message looks like
pic:file:nnn:
syntax error before 'token'
pic:file:nnn: giving up on this picture
where file is the name of the file being processed, nnn is a line number within that file, and token is a symbol in the pic language (often a keyword) near (usually just after) the error location.