Resistance in an AC circuit is called Impedance, also referred to as Z in formulas. Finding resistance is much different in AC then in DC. In DC circuits we simply use Ohm’s Law to calculate the resistance, but in AC we have a different formula that we use. Attached below is a good reference.
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The word impedance is used in many engineering disciplines to refer to an opposition to work done. In the context of electronics, it describes the combined effect of resistance (R), inductive reactance (XL) and capacitive reactance (XC) in an AC circuit, whether it occurs in a single component, or in a whole circuit. Since XL, XC, and R are almost always out of phase and the inductor and the capacitor have reactances that change depending on frequency, impedance is a way to combine them to perform math with.
A high impedance connection on a component is designed to function with low current. Think of a reset pin on an IC. Not a lot of current is needed, so it is an high impedance connection, while something like a LED would have a low impedance connection unless it is connected in a reversed bias. In a reverse bias, the PN junction will not allow for electrons to flow, so the connection becomes high impedance.
I hope this helps.
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