Questions & Answers about A tampa desta panela está na pia.
A is the feminine singular definite article, so it means the.
- A tampa = the lid
- Uma tampa = a lid
Here the speaker is talking about one specific lid, not just any lid.
Traditionally:
- desta = of this
- dessa = of that / of the one you mentioned
In everyday Brazilian Portuguese, though, essa/dessa are often used where a textbook might teach esta/desta. So in natural speech, many Brazilians would also say A tampa dessa panela está na pia.
Using desta is correct and clear; it just sounds a bit more precise or slightly more formal in many contexts.