What is meant by onmousedown?

Question

I used this code in Arduino but I don't how this onmousedown works and what is meant location.href=/?off13 in this code:

client.println("<input type=button value=OFF style='width:150px' onmousedown=location.href='/?off13;'>");
if(readString.indexOf('12') >0)//checks for 6
{
    digitalWrite(7, HIGH);    // set pin 7 high
    Serial.println("Led 7 On");
}

if(readString.indexOf('13') >0)//checks for 7
{
    digitalWrite(7, LOW);    // set pin 7 low
    Serial.println("Led 7 Off");
}

Answer 1

The onmousedown event occurs when a user presses a mouse button over an element.

<p onmousedown="myFunction()">Click the text!</p>

The onmousedown attribute is similar to the onclick attribute, but differs in that the event is triggered the moment the mouse button is pressed on the element, rather than at the point at which the mouse button is released (onclick is effectively a combination of onmousedown and onmouseup event on the element)

The href property sets or returns the entire URL of the current page.

location.href=URL