Please help, I with javascript for a checkbox

There is no "obviously" in programming. You have to explicitly layout exactly what you want to happen, and don't want to happen. Computers don't do nuance and innuendo. 

For example, I'm assuming that the script you posted is a MouseUp script on the checkbox, but you didn't specify, so I don't really know.  If it's not a MouseUp script, then any suggestions I make won't work. You must be explicit about the details. 

Here's an updated script that will set and reset the values, if my assumptions are correct.  

if (event.target.value!="Off") {

    // this code runs when the checkbox is checked
    this.getField("Previous Employer Name").value = this.getField("Company NameRow1").valueAsString;
    this.getField("Address 1_2").value = this.getField("Company AddressRow1").valueAsString;
}

else{

    // This code runs when the checkbox is unchecked

    this.getField("Previous Employer Name").value = "";

}

Try this:

if (event.target.value=="Yes") {