I'll be upfront about something before we get into this one. Nobody in this house plays clarinet or sax or oboe. Guitar and drums, that's our whole world. But we get asked about reeds constantly by parents whose kids just started woodwinds, and the honest answer is it's more confusing than it needs to be, so here's the plain version.
A reed is the thin strip that actually vibrates to make sound on a clarinet, saxophone, oboe, or bassoon. Different instruments need different reeds, a clarinet reed won't work on a sax and vice versa, the mouthpieces just aren't built the same.
The number on the box isn't a size, it's a stiffness rating
This trips a lot of people up. When you're buying reeds you'll see a number, usually something between 1 and 5, and it's easy to assume that's just picking a size like shoe sizing. It's not. That number is telling you how stiff or flexible the reed is. Lower number, softer and easier to get sound out of. Higher number, stiffer, harder to play but generally capable of a fuller, more controlled tone once you can actually handle it.
Oboe, English horn, and bassoon use words instead of numbers, soft, medium-soft, medium, medium-hard, hard, but it's the same idea underneath.
One thing worth knowing before you buy based on a number alone: that number isn't standardized across brands. A 2 from one manufacturer can feel noticeably different from a 2 made by someone else. So "my kid uses a 2.5" doesn't automatically transfer if you switch brands, it's more of a starting point than a guarantee.
What beginners should actually start with
Most band directors and private teachers point new players toward something soft, generally in that 2 to 2.5 range. There's a reason for that beyond just it being easier: a too-stiff reed for a beginner makes it genuinely hard to produce any sound at all, which is discouraging fast, especially for a kid who's already nervous about a new instrument. Go too soft on the other end and the tone comes out thin and weak, so it's not simply "softer is always better" either.
As the muscles around the mouth build up strength, which happens with regular playing, a stiffer reed usually becomes the better choice, better tone, better control, especially in the higher register. This isn't something to guess at on your own though. Your kid's teacher is going to notice when it's time to move up a step long before you would, since they're hearing the tone quality up close every week.
Filed vs. unfiled, and why it's not just a technical detail
You'll come across reeds labeled filed or unfiled, and it sounds like a minor manufacturing detail, but it actually changes how the reed plays. A filed reed tends to blow more freely and produces a slightly brighter sound, generally paired well with mouthpieces that already have some resistance built in. An unfiled reed gives you a darker tone with more natural resistance, and tends to pair better with mouthpieces that are already easy-blowing.
Which one's right depends heavily on the specific mouthpiece being used, not just the instrument in general, so this is genuinely one of those "ask before you guess" situations rather than something to figure out through trial and error if you can avoid it.
There's no universal right answer here
This is maybe the most important part, and it's also the least satisfying. There isn't one correct reed brand or strength that works for everybody. It comes down to the player, the mouthpiece, the style of music, even personal preference once someone's been playing a while. What works great for one kid in the same section might feel completely wrong for another.
If your kid's just starting out, don't overthink brand loyalty yet. Start soft, follow what the teacher recommends, and treat the first few months as figuring out what actually works for them specifically, not what worked for somebody else's kid down the street.
If you're standing in front of a wall of reeds with no idea what any of it means, that's a completely normal place to be. Happy to help point you toward a reasonable starting point, we just won't pretend we can tell you which one your kid's going to love best. That part's between them and their teacher.