Linear income can be best described as an income that is earned by trading your time for making money.
Simply put, the more hours that you spend working at your job, the more money you will make. And whenever you are not at work, you do not make money. That is basically what linear income is in a nutshell. You are trading a significant amount of time out of your life to receive a paycheck.
Linear income is by far the most common income of the poor and middle class because time is simply impossible to duplicate (or multiply).