high pass rc circuit .

why the output is reduced to lower amplitude.

The input is 10 volts DC and 1 volt AC.
The filter removes the DC.
The output is 1 volt AC.

another way to think about it.

The input signal is (10VDC and 1V at 1khz)
The filter removes anything below 10hz.
The output is 1V at 1khz.

Hi,

High pass filters remove the DC content of the wave because they filter out low frequencies and pass high frequencies. DC is considered a very very low frequency, so low it is called zero. That does not pass at all, while anything higher passes a little bit at least and much higher frequencies pass easily.
That's why it is called a "high pass" filter.

thanks for the nice explanation