The Stroke tab shows the stroke properties. e.g. the stroke width. You can define whether the stroke is solid or dashed and if dashed the stroke caps and joins.
Caps & joins
You can control the way endings of lines and angles between lines will behave. By using the Cap buttons, you can set the ending of a line as described below.
With the Join buttons, you can set the corner of an angle.
Creating and editing dashed lines
To create or edit a dashed line, click on the preview of the stroke on the Stroke tab. The Edit Dash dialog will be displayed. By entering values for the drawn dashes and the gaps in between, you can create custom dash sequences and apply them to paths.