Imagine some market where 10 companies sell bread and it costs $5. And lets assume that the costs are $4 and $1 is profit.
Then comes a company that makes its money on selling water and it is doing very well. It comes to a bread market and starts selling bread for $3.
The competition is quickly destroyed and the bread prices rise.
Edit: as a sibling comment mentioned: https://en.wikipedia.org/wiki/Predatory_pricing