Digital forensics is a forensic science where experts look at computer devices to help solve crime. It can also include mobile phones. The experts who do digital forensics are often called "analysts" or "investigators". When an expert is asked to look at a computer it is called an "investigation".
A digital forensics investigation happens when someone is blamed for a crime that includes using a computer. The expert will look for evidence about the crime. They will try to prove whether the person is to blame or not.
Sometimes investigations are used in disputes between companies and/or people (known as civil law). These may not involve a crime. Instead the expert is asked to find out information about a person or company by looking at their computer. There is a specific word used to describe this type of investigation, it is "eDiscovery".
Another use of digital forensics is called "intrusion detection". It happens after a hacker breaks in to a computer network. After the break in an expert is often asked to look at the networked computers to try and find out how it happened.