'Hit the Ground Running' means to immediately start an activity and work very hard and with full commitment.
Example of use: "As soon as the CEO approval of the new product came through, the entire office hit the ground running".
The usage of the phrase is associated with the invasion of Normandy (also known as 'D-Day') in World War 2. It was used to describe Nazi troops hitting the ground and running for cover after Allied ships invaded the shores. However, the first recorded use of the phrase was in the story 'King of the Liars', which was published in several newspapers in 1895: "I knew I had five more cartridges, so I hit the ground running and squatted low down..."