A voiced sound is category of consonant sounds made while the vocal cords vibrate.

All vowels in English are voiced, to feel this voicing, touch your throat and say AAAAH.  Feel that movement in your neck?  That is voicing.

Consonants can be either voiced or voiceless.  Some consonant sounds are made with the lips and tongue doing exactly the same thing but the voice is OFF or ON (vibrating).  This leads to many mistakes since the sounds look the same and sound similar.  Look at the following sound pairs that are made in the same manner with the tongue and lips but not the voicing.  Touch your throat as you say each sound to feel the difference.

Voiceless Voiced
P B
T D
K G
F V
S Z
CH J

Most ESL speakers really have a difficult time with voicing at the end of words.  Which can lead to misunderstanding.  For example, if you mean“rise” and you don’t use your voice, it sounds like “rice”.   If you mean“crab” and you don’t use your voice, it sounds like “crap”.  If you mean“as” and you don’t use your voice, it sounds like “ass”.