Understanding Path, stroke & fill in Adobe Illustrator
Paths are invisible lines that join the coordinates that define a shape. For a path to become visible it must have a stroke applied to it.
In more detail:
When you draw an object, you create one or more lines called a path. A path is made up of one or more curved or straight lines, known as segments. The start and end points for a segment is called an anchor point. An anchor point is a bending point to modify the line segment. A path can be open or closed. An open path has open-ended endpoints, such as a line, while a closed path has connected endpoints, such as a circle. Paths can have two types of anchor points: smooth points or corner points. A smooth point connects two curved segments to create a smooth line, while a corner point connects two straight or curved segments to create a path direction change. You can draw a path using both smooth and corner points. The outline of a path is called a stroke, which you can format with different characteristics. You can specify stroke weight (thickness), color, or a dashed pattern. The interior of an open or closed path is called a fill, which you can also format with a color or gradient. You can change the shape of a path by dragging its anchor point. A selected anchor point or endpoint appears as a solid square, while an unselected one appears as a white square.
Stroke & Fill
A fill is a color enclosed by a path. A stroke is a line of color that precisely follows a path. Here is a example of stroke and fill:
The blue star is showing the path, which has then had gray added as the fill and black as the stroke, around the original path. Below is the fill and stroke box and what it does.