The  here is the subscript operator, used to point out, by index, which character is being referred to. For example, if we have a string variable named <s>, containing "SoloLearn" then the s refers to the first character 'S', the s refers to the fifth character 'L', remember index starts from zero.
char S o l o L e a r n
(Look under the "Member and pointer operators")
strlen (short for 'string length') is a function that returns a value indicating how many characters there is in a string, in the case of string containing "SoloLearn", strlen returns 9, because there are 9 characters in the string.