This means that in the event that you die while your life insurance policy is being underwritten, then the life insurance company agrees to pay all or at least a portion of the proceeds of the death benefit for which you have applied to your beneficiary.
Once your life insurance policy is issued, the "In Force" policy then replaces the temporary agreement with the full life insurance coverage offerd by the policy for which you applied.
If you have certain health conditions, purchase certain types of life insurance policies, or do not wish to pay the first premium when your application is taken by the life insurance agent, your policy will go into effect after it is approved by the insurance company when the underwriting is completed.
The life insurance company will issue it with a few delivery documents to sign, and it goes into effect when those are received along with the payment for the first premium on your life insurance policy.