Suppose that the attorney the Metzgars hired agreed to represent them on a contingency-fee basis. What does that mean?

Ronald Metzgar placed his fifteen-month-old son, Matthew, awake and healthy, in his playpen. Ronald left the room for five minutes and on his return found Matthew lifeless. A toy block had lodged in the boy’s throat, causing him to choke to death. Ronald called 911, but efforts to revive Matthew were to no avail. There was no warning of a choking hazard on the box containing the block. Matthew’s parents hired an attorney and sued Playskool, Inc., the manufacturer of the block, alleging that the manufacturer had been negligent in failing to warn of the block’s hazard. Playskool filed a motion for summary judgment, arguing that the danger of a young child choking on a small block was obvious.