There were undefined references

This error appears when \ref{...} is used and LaTeX is unable to find the corresponding label. There are three ways in which this can happen.

There is a typo in \label{...}
There is a typo in \ref{...}
A label has been forgotten.

An example is shown below

\section{introduction}\label{introduction}
A typo when referencing the introduction would be \ref{intorduction}.

Another error is referencing a label which has never been defined such as \ref{section1}

Each of these will generate an error which looks like

main.tex, line 6

Reference `intorduction' on page 1 undefined on input line 6.

main.tex, line 6

Reference `section1' on page 1 undefined on input line 8.

main.tex, line 6

There were undefined references.

To avoid this, we must always make sure that when we use \ref{...}, that there is a corresponding \label{...}, and that the arguments of \ref{...} and \label{...} are the same.