Voiced means vibration: Put your hand on your throat and say "this" — you'll feel vibration during the ⟨th⟩. This is /ð/. Now say "think" — no vibration. That's the different sound /θ/.
The function word pattern
Almost all words with /ð/ are common function words — the small, frequent words that glue sentences together.
Simple rule: If the word is a common function word starting with ⟨th⟩, it's probably voiced /ð/: the, this, that, they, them, their, there, then, than, though, thus. If it's a content word (noun, verb, adjective), it's probably voiceless /θ/: think, thing, three, thick.
Words with /ð/ in the middle
Several content words have /ð/ between vowels — often in family terms and common words.
Noun vs verb pattern: Many noun/verb pairs differ by /θ/ vs /ð/: breath→breathe, bath→bathe, cloth→clothe, loath→loathe. The noun has voiceless /θ/, the verb has voiced /ð/.
/ð/ vs /θ/
Same spelling, different sounds! Both /ð/ and /θ/ are spelled ⟨th⟩. Compare these minimal pairs: breathe (verb, /ð/) vs breath (noun, /θ/), bathe vs bath.
/ð/ vs /d/, /z/, /v/
Learner tip: If your language doesn't have /ð/, don't substitute /d/, /z/, or /v/! They're different sounds. Practice with: this (not "dis" or "zis"), the (not "da" or "ze"), that (not "dat").