Greetings Antenna_ant:

What is impedance?

Impedance is the opposition to current flow and
contains both a real and an imaginary component,
in the form of (Real +/- j*Imaginary). In the case
of a pure resistance, the imaginary component is
0, and in the case of a pure reactance, the real
component is 0.

Why does it matter so much
in RF?

Impedance is important because mismatch
between connected components causes poor efficiency
(DC to RF conversion) and causes a perfectly level
input to exhibit rippling and/or slope at the output.

What is impedance matching?

Impedance mismatch
is when the output impedance of one component does
not exactly equal the input impedance of the next
component in line. Theoretically, maximum power
transfer between two components is achieved when
the load impedance is the complex conjugate of the
source impedance. For instance, if the source impedance
is 25+j10 ohms, then the load impedance must be
25-j10 ohms.

You can poke around on the RF
Cafe website and other websites for more detail.

- Kirt Blattenberger