Relaxed shorts suit anyone who wants to feel decent in warm weather without committing to the performance of it all, the tan, the pose, the tiny inseam. They're particularly good if your thighs are not the part of you that wants attention that day, or if you simply can't be doing with fabric climbing anywhere when you sit down. Think twice if you're short waisted and prone to pleats, because a baggy short with a lot going on at the front can add bulk exactly where you don't need it, and the whole thing starts to look like it's wearing you rather than the other way round.
If you own none, start with something in a mid weight cotton or a cotton linen blend, knee length or just above, with a waistband that sits on the natural waist rather than low on the hip. That one length does the most jobs: it works with a plain top and trainers for a daytime errand, it works with a linen shirt knotted at the front if the occasion is slightly nicer than that, and it doesn't read as beachwear the second you're somewhere with a roof. Relaxed shorts are a spring and summer item through and through, they lose their nerve the second the temperature drops, there's no version of this short that works under a coat without looking like an afterthought, so don't try to stretch the season on them. They're at their best on a hot pavement, on a train with no air conditioning, on the kind of day where anything fitted feels like an argument with your own skin. A good pair also travels well, which matters more than it sounds, because they pack flat and don't crease themselves into a disaster the way a tailored short can.
The difference between a pair that earns its hanger space and one that gets worn twice and binned is almost always in the fabric and the waistband. Cheap versions use a thin cotton that goes see through the first time you bend over in sunlight, and a waistband that's one flat piece of elastic with no structure, so by midday it's rolled itself into a rope. A properly made pair has a bit of weight to the cloth, a waistband with either a proper facing or a drawstring that actually holds its knot, and stitching along the pockets that doesn't pull at the seam after one wash. Check the hem too, a cheap short is often finished with a single thin line of stitching that unravels, while a better one has a doubled hem that keeps its shape and its length wash after wash.
Try a pair on and sit down in them before you decide, that's usually the whole test.