Toxic exposure means that a toxin (poison) touched or got into a person's body. That person has been exposed to the toxin.
When a person is exposed to a toxin, many different things about that person, the toxin, and the exposure affect toxicity. Toxicity is how the person reacts to the toxin, including what symptoms they may have.
A person can get exposed to a toxin accidentally or on purpose. People can also use toxins as weapons to try to kill other people (for example, during chemical or biological warfare).