Why it is written CS over any example image in chapter..

Answer 1

https://code.sololearn.com/W3uiji9X28C1/?ref=app

Answer 2

Your question is unclear. Read these (and what HrCoder linked to) and give it another try:
https://www.sololearn.com/discuss/333866/?ref=app
https://www.sololearn.com/discuss/1316935/?ref=app

Answer 3

Perhaps because you are studying C# ? CS -> CS[harp]. I think it is just a language indicator, so that you know the example code you are looking at is a code written in C#.