Rhizomes are underground (usually, although they can be above ground as well) stems that grow away from the mother plant. At intervals along the length of the rhizome, at the "nodes", the plant will produce a new set of roots and aerial stems. Plants with rhizomes do spread, but the rate at which they do so varies greatly.
Monardas generally don't spread all that fast and are fairly easy to deal with. If they start to crowd other plants, just take a shovel or digging knife and carve off the unwanted sections and either plant them somewhere else or throw them away. It won't hurt the rest of the plant.
