Criminal intent is the conscious decision to commit a crime. It is often used to determine what extent a party can be held accountable for a crime. There are different forms of criminal intent. For example there is straightforward premeditation, as in, a person directly plans in advance to commit a crime. Another example would be if a person decides to commit a crime spontaneously, but they still have knowledge that what they are doing is illegal. In any case, if criminal intent is established, there is a much greater chance that the person will be found guilty of the crime.
Criminal intent basically just means that the person who committed the crime is aware that what they are doing is illegal, and they do it anyway. This is opposed to committing a crime by accident, as in trespassing on land that a person didn't know was privately owned, or without intent, such as an insane person destroying property. Criminal intent is the mental aspect of "mens rea" which is the set of things needed to determine guilt in a criminal case.