When a line has loose spacing

Stet flags a line when the gaps between its words grow unusually wide. In a justified book every line is stretched to reach both margins, and sometimes the words on a line simply don't fit comfortably, so the spaces take the strain. Printers call the result a loose line, and a run of them produces "rivers": pale channels of white running down the page that the eye follows instead of the words.

It is a flag, not an error. Your book will print. The question is only whether that line looks right to you.

Why one line goes loose

Justification works by shrinking and stretching the spaces between words. A line goes loose when it has too few spaces to share the work:

A long word won't fit. The line has to end early, and the remaining words stretch to fill the measure. This is the most common cause.
A proper noun or foreign word can't be broken. Hyphenation dictionaries don't know Cansaladeria or Dumitrescu, so they never offer a break point.
The measure is narrow. A small trim size, a large body size, or generous margins leave fewer words per line, so each one carries more of the stretch.
A short last word is stranded. The line before a paragraph's end sometimes has to hold open for a single short word.

What to try

Work down this list. The first two are usually enough, and the last is a perfectly respectable answer.

Allow a hyphen in one word

If the line ends just before a long word, letting that word break will pull part of it up and close the gaps. In the editor, select the word, then choose Hyphenation → Choose break… and pick where it divides. You choose the point. Stet does not guess at the word's language or force a break.

This is the right fix when there is an obvious long word at the boundary. It is the wrong fix when there isn't: forcing a break into a short word, or into a name, looks worse than the loose line did.

Edit the sentence

Often the quickest real fix is a word. Cutting or adding a single short word earlier in the paragraph re-flows every line after it, and the loose line usually disappears. Authors do this routinely at proof stage. It is not a compromise, it is typesetting.

Change the measure

If loose lines are frequent rather than occasional, the cause is the page, not the sentence. Any of these give every line more words to work with:

a wider trim size
a smaller body font size
narrower margins in Advanced Page Layout

A book with many loose lines is telling you its measure is too narrow for its type size.

Turn hyphenation on

With hyphenation off, Stet can never break a word to rescue a line, so loose lines become much more common. If you have it off by preference, this is the trade you are making. It lives under Style → Print Settings → Paragraphs.

Keep it

One loose line in a chapter is invisible to a reader. Choose Keep as is and Stet stops flagging that line. Professional books contain loose lines. What they avoid is several in a row, which is what produces a visible river.

When a hyphen won't help

Some lines cannot be fixed by hyphenation at all, and it is worth recognizing them so you don't spend time trying:

The long word is already at the start of the next line. There is nothing on the flagged line to break.
The words are all short. Hyphenating a five-letter word gains almost nothing and reads badly.
The line ends a paragraph. A short last line is not justified at all. If it looks loose, the cause is the line above it.
It's a name you don't want broken. Choose Hyphenation → Never for this occurrence to keep it whole, and fix the line another way.

In those cases, edit the sentence or accept the line.

Finding your decisions again

Every hyphenation decision you make is listed under Hyphenation in the Structure sidebar, grouped by chapter. Click one to jump straight to the word. With invisible characters shown, a chosen break also appears in the editor as a small tick under the word, in the same style as a soft hyphen you typed yourself.