I found this sentence in Swift's String document (https://developer.apple.com/documentation/swift/string)
A string is a series of characters, such as "Swift", that forms a collection. Strings in Swift are Unicode correct and locale insensitive, and are designed to be efficient. The String type bridges with the Objective-C class NSString and offers interoperability with C functions that works with strings.
But, I can't understand this one hundred percent and I don't know where to start.
It basically just means that Swift strings are Unicode strings. A Swift string "character" is a character in a Unicode sense: a letter, an emoji, a combined letter-and-diacritic, whatever. A string can also be viewed not merely as a character sequence but as a sequence of UTF8, 16, or 32 code points. The "locale insensitive" stuff means they don't have a locale dependent encoding, as strings did in the bad old days before Unicode.
This is delightful but it has some downsides, most notably that strings qua character-sequence are not directly indexable by integers.