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What is Internal appearance of the lesion can be determined by the term: and explain in detail?
The internal appearance of a lesion can be classified into one of three basic categories:

Totally radiolucent
Totally radiopaque
Mixed radiolucent and radiopaque (mixed density).

A totally radiolucent interior is common in cysts, and a totally radiopaque interior is observed in osteomas. The mixed density internal structure is seen as the presence of calcified structures (white) against a radiolucent (black) backdrop.
Oral radiology White & Pharaoh 7th edition: page no 277