With extraction anchor we mean that part of the extraction command that is used to find the information to extract. Since the information is always inside a HTML element, the extraction anchor defines this HTML somehow.
If we have this web element:
<span class="price">US$ 899.00</span>
Assume we want to extract the price. So in this case the bold part is the extraction anchor. It never changes, even if the information (here: US$ 899) changes.
TAG POS=1 TYPE=SPAN ATTR=CLASS:price EXTRACT=TXT