Most languages have similar features, so I'm not going to focus on any language specifically as you didn't mention which language you're interested in.
A string is a sequence of characters, e.g.: "Hello" is a string that consists of the characters 'H', 'e', 'l', 'l', 'o'.
A string in most languages supports indexing, i.e. accessing specific parts.
E.g.:
myString = "Hello";
characterE = myString[1];
The above code gets the 1st character of the string "Hello", that is the character 'e'. In computer science indexing almost always starts from 0. So the very first letter is at index 0, the second is at index 1, etc.
If you're interested in language-specific features, share what language you'll use.