The chapter explains how the BNC marks up text files.
What is mark-up?
Mark-up is extra text added into a text file, other than the message itself.
<LETTER>
<GREETING>
Dear Svetlana
</GREETING>
<BODY_of_LETTER>
I hope you are well.
We are now living in Gloucestershire, in the countryside, not far from Minchinhampton.
...
...
</BODY_of_LETTER>
<LEAVE_TAKING>
All best wishes
</LEAVE_TAKING>
<SIGNATURE>
Mike
</SIGNATURE>
</LETTER>
All the bits in <ANGLE BRACKETS> are mark-up, labelling the structure of the text.