Shorten the lines
Paste a text and choose the length (40 characters by default): each longer line is truncated with «…».
What it's for
Every line longer than the length you choose (forty characters by default) is cut and closed with an ellipsis, which counts towards the total: no line comes out longer than the number in the field. It's for when a list has to fit into a narrow space and seeing all the entries matters more than reading them in full: a list preview, chart labels, a spreadsheet column, the titles of a rundown, a form field that only takes a few keystrokes. The result is a tidy column instead of lines that overflow, each by a different amount.
The cut is by characters, not by words
The count is in characters, so the cut can land in the middle of a word: with the length set to 13, «conventionally» becomes «conventional…». That's deliberate, because it's the only way to guarantee that every line is the same width, which is the whole reason for truncating. If what you need is a readable summary, an automatic cut isn't the tool for that job.
Characters are counted the way you see them: an emoji, a flag or an accented letter count as one, even when they take up two or more slots in memory, and they are never split in half, which would leave a little box instead of the picture.
Short lines are left alone
A line already within the chosen length stays identical, with no ellipsis: the dots only appear where something was actually removed, so you can see at a glance which entries are complete and which aren't. Empty lines stay too, and the number of lines never changes: what goes in and what comes out have the same shape.
Nearby tools
If the lines are too long because the text is all run together, it's worth putting it back in line first with Text on one line or cleaning it with Remove extra spaces. To sort the entries from shortest to longest, and see straight away which ones overflow, there is Sort lines by length. And if the limit you have to respect is a social network's, the exact count is done by Social character counter.