The words dense, dents sound the same but have different meanings and spellings. Why do dense, dents sound the same even though they are completely different words?
The answer is simple: dense, dents are homophones of the English language.
Slow to apprehend; thickheaded.
Having relatively high density.
Crowded closely together; compact: a dense population.
Hard to penetrate; thick: a dense jungle.
Plural form of dent.
Third-person singular simple present indicative form of dent.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").