F is the letter of the drive.
/P:5 says write over the entire disk 5 times.
You may be asked what the volume label is for that drive. Right click the drive and the first field listed under the General tab will be the label. You can also change the volume label here. In this case the volume label is "Storage".
Be prepared for a long wait. In this case I have a 500 GB HDD and after 14 minutes I have completed 2%.