Toilet valve into wall leak.
I think what they're telling you to do is try tightening the nut where the stem of the shutoff valve goes into the valve body. This holds the stem, and compresses the packing around the stem so it doesn't leak.
Also, check the compression fitting where the flexible line from the toilet goes into the shutoff valve, and also check to see if the water might be coming from the bottom of the toilet, where the supply line enters the tank. It could be leaking from there, and running down along the supply line, giving the appearance that it is leaking from the shutoff valve.
If needed, you can tighten the compression nut where the supply enters the toilet. Be very careful here, as most of these are plastic. It should be snug, just a little more than hand-tight.
Another thing you should do to determine whether you actually have a leak on the supply side of the shutoff valve is to close the valve completely, so that no water will flow into the toilet. Dry up the floor, and the valve and fittings, then wait a while to see if anything still gets wet.
If it does, then you probably have a leak on the supply side of the valve.
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