The stroke modifier almost creates a new shape along the edges of your main shape.
It's similar to Solidify Path but for Shape layer.
This modifier lets you:

• Trace the edges like a shape
• Create a snake effect along the edges
• Modify the aspect of the caps
• Dash the stroke
• Repeat the stroke

Note that by using this modifier, filling method is changing.

Thickness

Use it to modify the thickness of the stroke.

Trace

Trace the stroke and create a snake effect.

Warning: Placed over a Group of shapes , the trace option change depending of the blending mode of the differents shapes inside.
The option "Group behaviour" is also useful when placed over a Group of shapes a or a Text. It's quite like the option "shape index" of the Position in Shape mask:
The trace option will take all the shapes or characters and trace the stroke one after the other.

Stroke cap (Start/End)

Similar to Joiner, only available if Trace is enabled.
Modify the way caps of the stroke join the rest of the edges.
you can choose between 6 mode for caps:

• Fillet
• Chamfer
• Smooth (polynomial)
• Smooth (exponantial) : control this mode with the "Exponent" parameters
• Columns : control this mode with the "Count" parameters
• Stairs : control this mode with the "Count" parameters

Depending of the associated radius parameters, caps begin earlier/later along the shape.
If you want to go further with custom caps, use Instantiate!

Dash

Use it to replace the stroke by a dashed line.
Stroke modifiers have a Dash sub-object that can be customized with these parameters:

• Size : the length (in px) of each dash
• Gap Before : Define the margin (in px) before each dash
• Gap Before : Define the margin (in px) after each dash

Dash modifiers have 2 other variables:

• Offset : The offset of each dash
• Scale : The Scale (in %) of each dash. If you have set the size parameter of your dash to 100px and this parameter to 50%, every dash will appear with a length of 50px (50% of 100)

Note that you can add as many "Dash" sub-objects as you want by right-clicking on the Stroke modifier and choose Dash.

Repeat (Inside/Outside)

Repeat the stroke.

• Count : Number of repetitions
• Distance : Distance (in px) between each repetition
• Radius Multiplier : Thickness of the repetition. Depends on the thickness of the the stroke modifier