Plain English Meaning
A service where a controller watching radar tells you about other aircraft near you and may suggest a heading to stay clear, while you are flying by visual rules. You still have to look outside and avoid traffic yourself.
Full Definition
An air traffic control service in which radar-equipped facilities provide pilots flying under visual flight rules with information on observed traffic, advised headings to avoid that traffic, and limited safety alerts. The service is workload-permitting and does not relieve the pilot of the responsibility to see and avoid other aircraft.
Context Anchor
You may encounter this after canceling an instrument flight plan and continuing visually, when you want to stay in with for traffic help.