whats the meaning of substring

string input = "abcdef";

    // Get first three characters
    string sub = input.Substring(0, 3);

result is abc

int i = textBox1.Text.Length;
textBox1.Text = textBox1.Text.Substring(0, i - 1);

In your example Substring(0, i - 1) means,Start from the 0th index and total text length goes for the (i-1).

LIVE DEMO : .Net Fiddle

Please Read for more : C# Substring

UPDATE

Below code snippet gets first three characters of the 'input' string.It's like this.It starts from the 0th index and total length of the final output should be 3 characters long.

string input = "OneTwoThree";

// Get first three characters
string sub = input.Substring(0, 3);

OUTPUT

One

SubString returns a part of the string, by either

specified starting character position --> the end of the string

string a = "Hello World";
string b = a.SubString(6);
Console.WriteLine(b);

output:
World

specified starting character position --> a specified lenght

string a = "Hello World";
string b = a.SubString(0,5)
Console.WriteLine(b);

output:
Hello

http://msdn.microsoft.com/en-us/library/aka44szs(v=vs.110).aspx[^]

Consider a small example of mine, so that you will be able to relate with your code.

String MyEmail = "SanjuBollera@gmail.com";
int HowManyChracters = 5;
int FromWhatPosition = 13;
Console.WriteLine(MyEmail.Substring(FromWhatPosition)); //This returns a SUBSTRING "gmail.com"

Console.WriteLine(MyEmail.Substring(FromWhatPosition, HowManyChracters)); //The output is "gmail"

//The first parameter tells you from where should you start extracting the string
//The second parameter tells you how many characters you should extract from that substring.

substring is a method of a string it returns only the value between the range

string input = "OneTwoThree";

   
    string sub = input.Substring(0, 3);
    Console.WriteLine("Substring: {0}", sub);

Result:
one

string input = "OneTwoThree";

   
    string sub = input.Substring(4, 3);
    Console.WriteLine("Substring: {0}", sub);

Result:
Two

substring('yourstring',startIndex,endIndex)

More Info

We saw many examples concentrated on the Substring instance method with one or two arguments. With MSDN, we researched the Substring method. Finally we learned about Slice, Substring exceptions, and Substring performance.


Review:
Substring is useful.
It can help simplify your programs.
It has no significant performance problems
What is SubString

Thanks,
_RG

You should take the habit to go and see MSDN when it comes to using basic .NET objects (in that case, the String object):
String.Substring Method[^]

It will explain everything you should know about the Substring() method and its overloads.