The square
value indicates that at the end of each subpath the stroke will be extended by a rectangle with a width equal to half the width of the stroke and a height equal to the width of the stroke. On a zero length subpath, the stroke consists of a square with its width equal to the stroke width, centered at the subpath's point.
Example
<svg viewBox="0 0 6 4" xmlns="http://www.w3.org/2000/svg">
<path d="M1,1 h4" stroke="black" stroke-linecap="square" />
<path d="M3,3 h0" stroke="black" stroke-linecap="square" />
<path d="M1,1 h4" stroke="pink" stroke-width="0.025" />
<circle cx="1" cy="1" r="0.05" fill="pink" />
<circle cx="5" cy="1" r="0.05" fill="pink" />
<circle cx="3" cy="3" r="0.05" fill="pink" />
</svg>