SVG has a very cool and powerful element called <use>. It's pretty simple in concept. It finds another bit of SVG code, referenced by ID, and clones it over inside the <use> element.
The most basic use case would be: I've already drawn this shape(s) once on the page, and I want to draw it again someplace else. Go get that shape(s), and draw it again.
We can use that ability as the root concept for (drumroll!) and entire icon system! We can take all the shapes we intend to use on the page in one big SVG block. We'll wrap them all in a <defs> tag which is a semantic way to say "We're just defining these things in which to use later." It also makes sure they won't render (but you should also display: none; the <svg> itself.
That funky looking xlink:href attribute references an ID elsewhere. That ID could be on a any shape element, like a <rect> or <path>, or it could be on a group of elements like a <g>.
Best of all in the case of an icon system, it can be on a <symbol> element. In addition to being correct semantically (an icon is a symbol, no?), the <symbol> element can carry it's own viewBox attribute and accessibility information, like <title> and <desc> tags. This makes the implementation very easy (as shown above).
So you just need to make sure this is defined somewhere else in the document: