Simplified, its just the internal resistance of the output. An equivalent circuit would be just a resistor in series with the sourcing output pin.   

Any non-ideal source has some (parasitic/unwanted) internal resistance due to the physical layout of the source. In general it needs to be low compared to the load, because with increasing current, the voltage drop on the internal resistance increases and thus, the output voltage decreases.

I keep talking about resistance but you asked about output impedance. 

Impedance = resistance + reactance

Reactance is the opposition of a circuit element to a change of electric current or voltage, due to its inductance or capacitance. Since the output behavior is not entirely resistive, we talk about "output impedance".

What is output impedance of a pin?
How to change the red color for
- the titles
- the topics

How to adjust the position of the header1 and 2?

\documentclass{beamer}

\usetheme{CambridgeUS}


% Set Color ==============================

% Custom colors
\usepackage{xcolor}

% http://www.computerhope.com/htmcolor.htm
\definecolor{gold}{HTML}{FDD017}
\definecolor{deep sky blue}{HTML}{3BB9FF}
\definecolor{light sky blue}{HTML}{82CAFA}

\makeatletter
\definecolor{mybackground}{HTML}{82CAFA}
\definecolor{myforeground}{HTML}{0000A0}

\setbeamercolor{normal text}{fg=black,bg=white}
\setbeamercolor{alerted text}{fg=red}
\setbeamercolor{example text}{fg=black}

\setbeamercolor{background canvas}{fg=myforeground, bg=white}
\setbeamercolor{background}{fg=myforeground, bg=mybackground}

\setbeamercolor{palette primary}{fg=black, bg=gray!30!white}
\setbeamercolor{palette secondary}{fg=black, bg=gray!20!white}
\setbeamercolor{palette tertiary}{fg=black, bg=gold}
\makeatother
% Set Color ==============================



\title{Title1}
\subtitle{Title2}
\author{Author}
\institute{Institute}
\date{Date}


\begin{document}

\frame{

}

\section[Outline]{}
\frame{\tableofcontents}

\section{Header1}
\subsection{Header2}
\frame{\frametitle{Topic1}
%
HTML color codes are the \textbf{hexadecimal triplets} representing the colors red, green, and blue.
\vspace{0.25cm}
%
These color codes can be used to change the color of the background, text, and tables on a web page.
}
%
%
\end{document}


Title1 and 2


Topic1 and Header1 &amp; 2


Align text left or right, center

