Help new to c# and need explanation with substring

Hey everyone I just started coding but now I'm learning about .IndexOf and substring I just don't know how it works. Like for example I wanted to do Initials (Take the first letter of the first name[text box 1]and the 2 first letters of the last name[text box 2]) as well as in 1 single textbox the initials same as before but it's not the same so can anyone please help me with this code