Word expansion means the process of splitting a string into words and substituting for variables, commands, and wildcards just as the shell does.
For example, when you write ‘ls -l foo.c’, this string is split into three separate words—‘ls’, ‘-l’ and ‘foo.c’. This is the most basic function of word expansion.
When you write ‘ls *.c’, this can become many words, because the word ‘*.c’ can be replaced with any number of file names. This is called wildcard expansion, and it is also a part of word expansion.
When you use ‘echo $PATH’ to print your path, you are taking advantage of variable substitution, which is also part of word expansion.
Ordinary programs can perform word expansion just like the shell by
calling the library function