The ByVal keyword indicates that an argument is passed in such a way that the called procedure or property cannot change the value of a variable underlying the argument in the calling code.

The ByRef keyword indicates that an argument is passed in such a way that the called procedure can change the value of a variable underlying the argument in the calling code.

Example

Public Class Form1
    Inherits System.Windows.Forms.Form
    Dim x, y As Integer

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        x = 20
        y = 30
        MsgBox("original value=" & x & "  " & y)
        change1(x)
        change2(y)
        MsgBox("changed value=" & x & "  " & y)
    End Sub
    Sub change1(ByVal x As Integer)
        x = x + 10
    End Sub
    Sub change2(ByRef y As Integer)
        y = y + 10
    End Sub
End Class

Output
original value = 20,30
changed  value = 20,40

By value means a variable or object is having copy of value and by reference mean having only address of value.