Voicing involves using the voice box while pronouncing a letter. It is the difference between k and g, between t and d, between s and z. Some languages have only one of a voicing pair, but others will have both. English has both a voiced and unvoiced “th” sound, but many other languages that have a “th” sound only have one of them.
In this video, Artifexian gives an overview of how voicing plays into the phonology of a language you want to create and what you need to consider when constructing your language. A good way to keep track of the sounds of your language is by looking at the IPA chart which shows all the possible sounds of human languages.