For that image (assuming it's a vector image) it's probably a white object on top of a blue object, with what it looks like is a gradient fill. Since the white object is offset from being centered over the blue one, it looks like a path that is smaller in one place than in another.
However, there IS a way, actually a couple of 3 ways, to draw a path with a variable width. The easiest to use is probably the Power Stroke LPE (live path effect). Another way is using Pattern Along Path, which comes as an extension or LPE. I recommend the LPE.
If you draw with the Pen or Pencil tool, and use Triangle In or Triangle Out from the Shape dropdown menu (on the control bar), the path automatically has the power stroke LPE applied to it. Or you can draw a regular path with Pen or Pencil, and apply the power stroke using Path Effects editor (near bottom of Path menu).
For the power stroke, after you apply the LPE (or draw the path with Pen or Pencil, as described), switch to the Node tool. Regular nodes are gray. But look for a pink node. If you drag the pink node, it adjusts the width of the line.
If you use Pattern Along Path, you have to draw a custom pattern, such as a triangle, which is applied to the path through the Path Effects editor. If you need to adjust the width, you have to edit the pattern. So power stroke is much easier to use. Manual info on pap:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Paths-LivePathEffects-PatternAlongPath.htmlThere's no info in the manual yet, about power stroke, since it's new for 0.91 version (and the manual STILL has not been updated for 0.91). But there's some info in the wiki about it:
http://wiki.inkscape.org/wiki/index.php/Release_notes/0.91#PowerStroke
