When is this said? And what does this generally mean? Thank you.
This is called Tefilas Haderech- lit. The Travel Prayer, and it is said when a person travels between cities that are distanced from each other (without being build up in between) about 2 miles (3.8 -4.6 km.). It is said after leaving the first city, (after traveling approx. 20-25 yards after the last houses of the city.
The idea of Tefilas Haderech is to ask G-d for protection against the various dangers of the road, and as a side point once we are beseeching G-d for protection we also ask him to give success for the reason that we are traveling, i.e. business, meeting etc.
After the actual prayer, there are a few verses that serve as a protection while traveling.
It might be helpful for you to get siddur with English translation, such as Artscroll or Metzudah Siddur. This way you can know the meaning of the prayers, blessings etc.
Shulchan Aruch O:CH 110-6,7.