ls is a command to lists computers files in Unix and Unix-like operating sistems. When invoked without any arguments, ls list the files in the current working directory.
An asterisk (*) is a linux wildcard, you can use it to match one or more occurrences of any character, including no character.
.c is the extension for a C file, and a C file is a source code file for a C or C++ program.
So, if we type ls *.c the output would be a list of all the C files (with the .c extension) in our current working directory.