Q: What is a matched filter?

Q: Why is matched filter also called correlator ?
A matched filter is used to detect the presence of some known signal.
Its impulse response is the timer reversed version of the template signal.
Basic fact:  mathematical operation of a filter is convolution with its impulse response;

The output signal is obtained by convolving an input signal with the impulse response of a filter.

Convolving unknown signal with matched filter becomes correlation of the signal. That’s why matched filter is also called correlator.
This is the optimal filter to maximize the signal-to-noise ratio.