Explanation and summary
The second plague of the 10 plagues that the God of the Israelites sent as judgment upon Egypt, through Moses, was frogs covering the land, as recorded in the book of Exodus. In this plague, frogs come out of the water and cover the land. Pharaoh's magicians duplicate this plague. (It is the last one they are able to reproduce.) The Egyptian gods this plague insulted include Heqt (associated with fertility and childbirth) and perhaps others as well.
Frogs are very abundant just after the high Nile when the waters begin to recede. Spawn in the mud is hatched by the sun, and the marshes are filled with myriads of these creatures. The frog was the hieroglyph for myriads. The frogs usually remain in the marshes, but in this case they came forth to the horror and disgust of the people. "Frogs in the houses, frogs in the beds, frogs baked with the food in the ovens, frogs in the kneading troughs worked up with the flour; frogs with their monotonous croak, frogs with their cold slimy skins, everywhere - from morning to night, from night to morning - frogs."
The frog was also associated with Divinity, was the symbol of Heqt, a form of Hathor, and seems also at times to have been worshipped as divinity. This plague created such horror that thus early Pharaoh came to an agreement (Exodus 8:8-10). A time was set for the disappearance of the frogs that he might know that "there is none like unto Yahweh our God," but when the frogs were dead, Pharaoh hardened his heart (Exodus 8:15 ). In this plague "the magicians did in like manner with their enchantments" (Exodus 8:7 ). Frogs were plentiful, and it would not seem to be difficult to claim to have produced some of them.
Biblical Text (KJV)
1 And the Lord spake unto Moses, Go unto Pharaoh, and say unto him, Thus saith the Lord, Let my people go, that they may serve me.
2 And if thou refuse to let them go, behold, I will smite all thy borders with frogs:
3 And the river shall bring forth frogs abundantly, which shall go up and come into thine house, and into thy bedchamber, and upon thy bed, and into the house of thy servants, and upon thy people, and into thine ovens, and into thy kneadingtroughs:
4 And the frogs shall come up both on thee, and upon thy people, and upon all thy servants.
5 And the Lord spake unto Moses, Say unto Aaron, Stretch forth thine hand with thy rod over the streams, over the rivers, and over the ponds, and cause frogs to come up upon the land of Egypt.
6 And Aaron stretched out his hand over the waters of Egypt; and the frogs came up, and covered the land of Egypt.
7 And the magicians did so with their enchantments, and brought up frogs upon the land of Egypt.
8 Then Pharaoh called for Moses and Aaron, and said, Intreat the Lord, that he may take away the frogs from me, and from my people; and I will let the people go, that they may do sacrifice unto the Lord.
9 And Moses said unto Pharaoh, Glory over me: when shall I intreat for thee, and for thy servants, and for thy people, to destroy the frogs from thee and thy houses, that they may remain in the river only?
10 And he said, To morrow. And he said, Be it according to thy word: that thou mayest know that there is none like unto the Lord our God.
11 And the frogs shall depart from thee, and from thy houses, and from thy servants, and from thy people; they shall remain in the river only.
12 And Moses and Aaron went out from Pharaoh: and Moses cried unto the Lord because of the frogs which he had brought against Pharaoh.
13 And the Lord did according to the word of Moses; and the frogs died out of the houses, out of the villages, and out of the fields.
14 And they gathered them together upon heaps: and the land stank.
15 But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them; as the Lord had said.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).