What is the meaning of Lorem ipsum?

Literally it does not mean anything. It is a sequence of words without a sense of Latin derivation that make up a text also known as filler text, fictitious, blind or placeholder

Why is Lorem Ipsum Dolor used?

The Lorem Ipsum text is used to fill spaces designated to host texts that have not yet been published. They use programmers, graphic designers, typographers to get a real impression of the digital / advertising / editorial product they are working on.