What is alignment in a word processing document?

Text alignment means side to which the text (lines, paragraphs) are aligned to.

Usually the title of a paper is in the middle of the line. This is called 'Center Align.'

The text on this page is 'Left Aligned.' Sometimes there is a picture on the right side of the page and the text flows around it. The picture might be 'Right Aligned.' Text about the picture might also be 'Right Aligned.'

Sometimes formal papers have margins on both right and left that are even, this is called 'Justified.'

Since we write from the left to right, we have left text alignment and all lines start in the same place on the left side of the document and end on the right side in different positions.

Arabic alphabet writes from right to left so they use right alignment.