This simply wires up an event - i.e. it says
"When the Click event of button1 is fired, execute the code in the method
button1_Click"
Otherwise, you would click on the button all day long, but nothing special
would happen.
In some languages (e.g. VB6), wiring up events is simply a case of writing a
method called (for example} {object}_{event}, but in C# you have to tell it
explicitely.
Don't worry about remembering the complex syntax; if you use VS2005, as soon
as you type += (next to an event), it suggests the next bit of code for you;
in fact, in .NET 2 even this is overkill, as you could just type:
button1.Click += button1_Click;
Of course, you have to have a function called button1_Click that meets the
required signature for the event... (which is done automatically if you
allow VS2005 to write the method stub for you)
Marc