DC resistance seen by a terminal?
The diagram is incomplete, in the sense that the left end of R1 is presumed to be connected to a voltage source.

Also, the output of the opamp is considered a voltage source, and the input of the opamp is considered to be a current source &mdash; a tiny one that represents the input bias current of the opamp.

When analyzing a network that contains multiple sources, you use the principle of superposition: Consider each source one at a time, replacing the other sources with short circuits (voltage sources) or open circuits (current sources).

Therefore, when analyzing the current source that represents the input bias current, you short the left end of R1 to ground, and also short the right end of R2 to ground. Now you have a current source connected to two parallel resistors.

In the second diagram, the capacitor blocks the DC current, so even though the left side of the capacitor is connected to ground, no current flows through R1 and you're just left with R2 connected to ground as the "load" on the current source.