A biochemical penetrant is a chemical that increases the ability of a poison to apply its toxic effect to a living organism.
Typically, the term penetrant when used for a biochemical agent, relates to an agrichemical that is used with a weedkiller or fungicide.
[1] The term seems to be used in relation to agrichemicals within English speaking countries rather than North American.
When mixed with a weedkiller (normally as an aqua solution) the penetrant chemical causes a plant to absorb the poison in a more effective manner and so succumb more readily.
Penetrants are most often used against plants that would otherwise be able to resist the weedkiller.