The "look" command line tool is used to search for lines that begin with a specific string. It is primarily used to find words or strings in sorted files more efficiently. When executing the "look" command, you need to provide a string or word as an argument, and it will look for lines that start with that specific string. It can only search for single words and does not support regular expressions. The "look" command requires the input file to be sorted in ascending order, as it uses a binary search algorithm to locate the lines. It returns all the matching lines found in the file, ignoring case sensitivity. This command can be handy when searching for specific entries or information in large sorted files more quickly and efficiently.