Disability insurance is an insurance that would pay you a monthly sum in the event that you are injured or disabled and cannot work.
In Japan it is known as 就業不能保険.
In Japan if you are paying the National Health Insurance and Pension, the government will also support you somewhat in the event of disability, however this may not be enough to live on, especially if you have a family.
Since you are much more likely to become disabled than die in an accident, disability insurance is important but more expensive than life insurance.
You can expect to pay approximately 1500-2000 yen per month for every 100,000 yen of disability insurance.
You will also need to provide evidence of your current income when applying.