Rest assured, it doesn't mean that the rocks can float!
Float is a term use to describe loose chunks of rocks that have broken free of the rock formation they originally formed in, and often have travelled some distance, either carried by a stream, or by rolling downhill.
Float is often very useful for finding interesting rock formations - look for the pieces, and then follow the trail of rocks as best as you can until you find where they originally came from.
Unfortunately, it's often hard to know just how far a piece of float may have travelled before you found it. Did it break off the cliff five yards upstream and get carried down last winter? Or was it carried three hundred miles by a glacier that melted away thousands of years ago?
How can you tell?
The first thing to look for is how well-rounded the piece is. If it's very smooth, it has likely travelled a long way in water, polished by other rocks and debris as it went along. If it has sharp, jagged edges, it's more likely to have come from someplace nearby.
If there are many other rocks around that look similar, then you are probably very close to the source. Look up - uphill, upstream, upriver - and if you find it, give yourself a big pat on the back. Well done!