What exactly does this converter change?
For every stroked shape it finds, it moves the stroke color into the fill attribute and removes the stroke, stroke-width, and related attributes, so the shape renders as a solid fill instead of an outline.
Does it turn a stroke into an outlined path with the same thickness?
No. A true stroke-to-outline conversion needs path-offset geometry, which this tool does not attempt. It performs the common, useful case of swapping stroke paint for fill paint and tells you how many shapes it changed.
Is my SVG uploaded to a server?
No. The markup is parsed and transformed entirely in your browser, so your SVG never leaves your device.