What is forbidden in the second commandment?

Answer

The second commandment forbiddeth the worshipping of God by images (; ) or any other way not appointed in his word ().

Read the 3 proof texts (KJV)

Deuteronomy 4:15-19: ¹⁵Take ye therefore good heed unto yourselves; for ye saw no manner of similitude on the day [that] the LORD spake unto you in Horeb out of the midst of the fire: ¹⁶Lest ye corrupt [yourselves], and make you a graven image, the similitude of any figure, the likeness of male or female, ¹⁷The likeness of any beast that [is] on the earth, the likeness of any winged fowl that flieth in the air, ¹⁸The likeness of any thing that creepeth on the ground, the likeness of any fish that [is] in the waters beneath the earth: ¹⁹And lest thou lift up thine eyes unto heaven, and when thou seest the sun, and the moon, and the stars, [even] all the host of heaven, shouldest be driven to worship them, and serve them, which the LORD thy God hath divided unto all nations under the whole heaven.
Exodus 32:5,8: ⁵And when Aaron saw [it], he built an altar before it; and Aaron made proclamation, and said, To morrow [is] a feast to the LORD. ⁸They have turned aside quickly out of the way which I commanded them: they have made them a molten calf, and have worshipped it, and have sacrificed thereunto, and said, These [be] thy gods, O Israel, which have brought thee up out of the land of Egypt.
Deuteronomy 12:31-32: ³¹Thou shalt not do so unto the LORD thy God: for every abomination to the LORD, which he hateth, have they done unto their gods; for even their sons and their daughters they have burnt in the fire to their gods. ³²What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.