A sword frog is the strap of leather that’s hung from the belt and used to secure a sword to someone's side. Historically, sword frogs were worn on a man's left side so that he can easier access his weapon in case he has to draw it fast. They’re typically made to hold any single handed sword and fit any belt or strap that they would be hung from.
Typically, sword frogs are made from leather, however, in current years, metal or chain has been used for the purposes of adding an appealing look to them. The frog would be made from a simple leather strap that slips around the belt of the wearer. From the end of the strap, a loop would hand where the sheath of the sword the wearer carries would attach. Sword frogs are made to hold the weight of typically any one handed steel sword, however there were frogs made that were able to carry two or more swords for knights who chose to carry more than one weapon. The frog was an invention that would insure versatility. Without it, each sword sheath would have to be custom made for the belt of the wearer. The frog made it where any sword could fit any belt.
The sword frog was generally hung from the man's left side and his sword was fit into it. The frog made it easier for him to draw his weapon faster in case of a surprise attack. The frog and sword were worn on the left side because most men are right handed and it is easier to reach across the body to draw your weapon than to reach on the same side. It has been said that the reason women stand on the right side of the man is because the positioning of the sword. He’d be able to draw his weapon and protect her without any problems.
Present Day Sword Frogs
Since there aren't many people carrying around swords nowadays, the sword frog has taken on other uses. They’re used for cosplay to hold mock weapons and add to the persons costuming. They’re also used in games known as larps, or live action role playing games, where they serve the purpose to hold foam or latex weapons for the person playing a character. Most frogs today are more ornamental as they’re used more for the look than for the actual function of using a heavy sword. People add chain, metal loops, and even jewels to modern day frogs to make them flashier.
Source: Sword Frogs: Past and Present