It means, in your query you use more than table, which have the same field's name, so you must tell the engine which field do you mean. Therefore you have to use the TableName the a period then the field's name you mean.
Hi scjohn and "Welcome to the Forum
Go to your Query in DesignView
Locate the FieldName that is the one in your ErrorMessage
Click on the drop down arrow in the QBE grid where it says Table
Once that drops down, select the correct Table that particular field is coming from. And that should do it.