The motte and bailey fallacy is an informal fallacy in which two separate forms of an argument are given and are switched between as necessary in an attempt to confuse or frustrate counterarguments.
The motte and bailey was a form of defensive structure in which a protected walled area surrounded a much stronger keep. Hopefully, the bailey would be enough to keep out attackers, but if it failed the defenders could retreat to the much stronger motte to protect themselves.
A motte and bailey argument works the same way; Bob claims "weak claim is true!", Anna says "weak argument is weak", and Bob spends some time explaining "strong argument is true; you must agree to that!" Anna agrees that the strong argument is true, and Bob walks away confident that he has defended his weak argument.
Theoretically this is a particularly weak form of argument; the motte is easy to defend, but is not an argument that anyone really cares too much about. You can counter it simply by shouting loudly "what about the bailey!?" until the other person breaks down crying. In practice, the people using this form of argument are very invested, and are happy to repeat the motte ad infinitum. This is a terrible way to convince other people that you are right, but an excellent way to convince yourself that you are right, making motte and bailey arguments common in all contexts where having wrong opinions does not get you killed.
Common clues that a motte is coming into play include references to faith, patriotism, tradition, perversions, and/or Hitler. However, these are low-level mottes, and it is also common to see more subtle mottes in nutrition, economics, philosophy, and pretty much everything else.