'To frap' is a rather archaic term meaning to bind tightly; in modern English you will rarely hear it used outside of nautical (or Biblical) circles.
Frapping lines are used to secure things on a ship during a storm; they may reinforce the rigging, tie down large items on the deck (e.g., lifeboats), or be used to bind furled sails and halyards to keep them from flapping and banging in the wind ('frap the halyard' is one of the most common modern usages of 'frap').
However, frapping is also commonly recognized among fans o the Bible and/or Greek and Roman shipping. While no common translation of the Bible uses the word frapping, Acts 27:17 refers to 'undergirding the ship' (in some plain English versions they change this to "they put cords under and round the ship" (BBE). As a result, most Google results for 'frapping' are Bible commentary -- an easy way to spice up an otherwise dull narrative.
Frapping was important in Greek and Roman ships in particular, as they were built out of light softwoods such as pine, and were optimized for speed over calm waters. Additionally, these woods absorbed water quickly, and ships were often pulled up on shore when not in use, allowing them to dry out. They were also carvel built, with the planks connected by pegs and glue (or, before this, sewn together with ropes). For these various reasons, ships didn't handle storms well, and changing the design to handle them better was neither easy nor the primary concern.
So, when a large swells might raise the boat so that it was suspended on the peak of a wave, leaving the end hanging out into the air (or, perhaps worse, picking up the two ends and leaving the center unsupported), one method of reinforcing the hull was to wrap ropes or chains around the ship lengthwise, from stem to stern to provide extra tension and keep the ship from flexing. Some have speculated that some ships may have had these cords permanently in place, loosened but ready to be tightened by a windlass. Unfortunately, as these ships tend not to be preserved, we have very little information on the details of Greek and Roman rigging.