First, how DEEP is the crack? If it's merely in the surface, it's called a "grass crack" and is more of a cosmetic issue than structural problem. It usually occurs when the hoof was super saturated and then dried over an over (lots of water crossings on a trail, lots of baths or standing in mud, etc) . Try to keep your horses' feet a little drier and it will go away as it grows out. DO NOT apply hoof oils (these can trap excess moisture in the tiny fissure, along with bacteria that can exaggerrate the prob) . As far as overall hoof healthy goes, keep on riding, won't hurt a thing.
If your horse is lame on that foot, get a farrier out ASAP , esp if it spreads visibly as the horse puts weight on that foot, or if blood starts to seep out as it works it way upwards of the hairline. But most likely, it's not that dramatic.
To check the depth (seriousness) of the crack, (since your horse is barefoot, should be easy to figure out.) Pick up his hoof. Look at the wall from the bottom and see if that crack is all the way through the thickness of the wall, or if you can even see the crack from the sole level.
If it doesn't go through the thickness of the wall, prognosis is great. If it DOES to through, it's still not a life and death issue. Typically, for a real crack that goes through the wall, it indicates that a trim is needed and the crack is usually tighter and shallower as it goes up, tapering into a surface crack.
A farrier/trimmer will trim the foot (excess length is likely what caused it begin with). Rasping a good bevel through the hoof wall will prevent cracks, and sometimes a notch is made where the crack is reaching ground level to take that weak section out of weight bearing to keep the crack from continuing, though that may not be necessary. Also, when finishing the hoof, making sure the wall is rasped to an even thickness all the way around, and the toes aren't "square" in appearance (squared breakovers lead to toe cracks, in my experience) .
If your horse doesn't usually have crack issues, it's likely a shallow one that can be fixed pretty easy by any farrier/trimmer.Shoes aren't necessary to stablize anything, as you may be told. I can grow out severe cracks barefoot and the horse stays in work quicker than I ever could with shoes. A better trim is usually all that's needed, and stay on that routine schedule (6 weeks).