If something is case-insensitive, it does not distinguish between uppercase and lowercase characters. It is the opposite of case-sensitive, in which the case of each letter matters.
Most search engines are case insensitive, meaning the case of the keywords does matter. For example, searching for "Microsoft Word" and "microsoft word" will return the same results. Case insensitive functions are able to match strings regardless of letter case.
Usernames are also typically case-insensitive. For instance, when logging in to a website, the username field of the login is not case-sensitive. The password field is. So, while capital letters usually don't matter when entering your username, they are important when entering your password.
Sorting algorithms are usually case-insensitive, meaning they treat "A" the same as "a." However, some functions use the ASCII value of each character, in which all uppercase letters come before all lowercase letters. In an ASCII sort, C comes after A, but before b.
When sorting text values in a database, the sort order is determined by collation, a rule for sorting the data. The default collation is typically case-insensitive and based on the text's character encoding, which assigns a numeric value to each character. MySQL and other database engines support multiple collations, such as latin1_general_cs (case-sensitive) and latin1_general_ci (case-insensitive). While it is rare to use a case-sensitive — or "cs" — collation, it is possible to override the default "ci" collation if necessary.