what is use of out keyword in C#?

The out keyword causes arguments to be passed by reference. This is similar to the ref keyword, except that ref requires that the variable be initialized before being passed. To use an out parameter, both the method definition and the calling method must explicitly use the out keyword.
or
Out signifies a reference parameter. Sometimes methods must return more than one value and not store class state. Out fills these requirements. With it we pass parameters whose changes are realized in their calling methods.
Example:--

using System;

class Program
{
    static void Main()
    {
    bool period; // Used as out parameter
    bool comma;
    bool semicolon;
    const string value = "has period, comma."; // Used as input string

    TestString(value, out period, out comma, out semicolon);

    Console.WriteLine(value); // Display value
    Console.Write("period: "); // Display labels and bools
    Console.WriteLine(period);
    Console.Write("comma: ");
    Console.WriteLine(comma);
    Console.Write("semicolon: ");
    Console.WriteLine(semicolon);
    }

    static void TestString(string value, out bool period, out bool comma, out bool semicolon)
    {
    period = comma = semicolon = false; // Assign all out parameters to false

    for (int i = 0; i < value.Length; i++)
    {
        switch (value[i])
        {
        case '.':
            {
            period = true; // Set out parameter
            break;
            }
        case ',':
            {
            comma = true; // Set out parameter
            break;
            }
        case ';':
            {
            semicolon = true; // Set out parameter
            break;
            }
        }
    }
    }
}

Output

has period, comma.
period: True
comma: True
semicolon: False