/Helvetica-Bold findfont
10 scalefont
setfont

sets the font in your current graphics state to be Helvetica-Bold. You have a limited choice in general:

/Helvetica-Bold, /Helvetica, /Times-Roman, /Times-Oblique, /Courier, /Symbol are some of the possible names. A more complete list is in the Reference Manual.

The next line in this fragment of code scales the font's nominal height to be 10 of the current units. Then finally the font at that scale is chosen to be the current font. You must choose some font in order to get text displayed.

There is no default font choice.