“The use by states and localities of criminal laws to go after undocumented immigrants simply for being undocumented is generally unlawful, because the federal government has sole power to regulate immigration. For example, in 2005, two New Hampshire sheriffs arrested and prosecuted several undocumented immigrants for “criminal trespass,” accusing them of being in New Hampshire unlawfully solely because they were undocumented. The state court that heard the case concluded(PDF) that the state law could not be used in that way because only the federal government can regulate immigration”.

WHAT DOES THIS MEAN IN YOUR OWN WORDS