question about the make manual

[ali hagigat]

4.4.5 Directory Search and Implicit Rules
For example, when a file ‘foo.o’ has no explicit rule, make considers
implicit rules, such
as the built-in rule to compile ‘foo.c’ if that file exists. If such a
file is lacking in the current
directory, the appropriate directories are searched for it. If ‘foo.c’
exists (or is mentioned
in the makefile) in any of the directories, the implicit rule for C
compilation is applied.
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If ‘foo.c’ exists (or is mentioned in the makefile) in any of the directories....

I copied an extract of the manual in the above.
How foo.c is metioned in the makefile?!! Could you please write an example?