When most people produce printed documents using a computer, they usually use software such as Word (part of Microsoft Office) or Writer (part of Open/LibreOffice) or similar–word processing software. SILE is not a word processor; it is a typesetting system. There are several important differences.
The job of a word processor is to produce a document that looks exactly like what you type on the screen. SILE takes what you type and considers it instructions for producing a document that looks as good as possible.
For instance, in a word processor, you keep typing and when you hit the right margin, your cursor will move to the next line. It is showing you where the lines will break. SILE doesn’t show you where the lines will break, because it doesn’t know yet. You can type and type and type as long a line as you like, and when SILE comes to process your instructions, it will consider your input (up to) three times over in order to work out how to best to break the lines to form a paragraph. Did we end two successive lines with a hyphenated word? Go back and try again.
SILE – What is SILE? http://sile-typesetter.org/what-is