Please see this answer to understand what is an extended UC.
UML, include, extend relationship
Extension point is defined in the extended UC and specifies how/when exactly the extending UC can be invoked. As a UC is typically described by scenarios and scenarios have steps, you usually describe the extension point as steps in which the extension can occur.
In the example I gave you, "Place order" is the extended UC and "Print Order" is the extending UC.
Let's suppose the following scenario of the "Place Order":
1. User fills the form
2. User confirms the input
3. System validates the input
4. If ok, System registers the Order
5. If ok, System displays the confirmation message
- Extension point might be defined for the step 5 (only successfull Order can be printed).
- Alternativelly it can be defined for the range of steps between 2-5 (Order might be in the DRAFT or COMPLETED state)
Another nice way to specify extension points is via activity diagram (if you want to avoid textual scenario descriptions).