If you keep adding more details and refinement, you create a higher fidelity wireframe. There is no sharp division line between low and high fidelity; they are two extremes. The highest fidelity wireframe would look exactly like a screenshot of the finished product. For practical purposes, high fidelity wireframes don't use sketchy rendering. Instead, sharp lines, well-defined boundaries, layouts, and alignments are used. High-fidelity wireframes are usually in color, instead of grayscale, and when available, branding elements could be used instead of placeholders.
They take considerably more time and resource investment, and generally, they are harder to create. Arguably, more skill is needed for them, and often high fidelity ...
