The message relates to a particular make, style, face and size of font for which the font metric information is not available. The name (in this case pplr7t) is a shortform of the longer name and is referred to as a 'Berry name'. You can find how the name is constructed in the file
fontname.pdf' (texdoc fontname.pdf` on a TeX Live system). In this case the request is for an Adobe Palatino font ('p'=adobe, 'pl'=Palatino, 'r'=regular Roman).
The Berry name is formed by (quoting from
Here is the basic scheme (the spaces here are merely for readability):
S TT W [V...] [N] [E] [DD] where
S represents the supplier of the
TT represents the typeface name.
W represents the weight.
V. . .
represents the variant(s), and is omitted if both it and the width are
Many fonts have more than one variant.
N represents the
encoding, and is omitted if the encoding is nonstandard. Encodings
are subsumed in the section on variants (see Section 2.4 [Variants],
E represents the width (“expansion”), and is omitted if it
DD represents the design size (in decimal), and is omitted
if the font is linearly scaled.
The lack of the font can be because it is not installed on your system but requested (either by a package or by your own commands) or because the font is installed but the particular combination of face, weight, size, etc. is not available (e.g. many fonts do not support semi-bold and hardly any support italic small capitals) or because what is looked for is installed but there is a problem with the TeX font metrics file (.tfm) or the font map file (.map), or because the commands
updmap-sys were not run or not run correctly when the font was installed (these are needed to correctly set up a updmap.cfg file that lists all the font maps available to the user or on the system). There are many questions and answers on the site covering these specific points in detail.