To wish somebody a safe and pleasant journey, that everything may go well.
Some Chinese feel it is improper to use this phrase when the parting friend will travel by flight, since this phrase includes the word 风 (fēng - "wind"). Instead you may safely use the phrase 一路平安 (yí lù píng ān)
By the way.. Did you notice that this phrase is part of 3 different groups so far? It just goes to say, that it is very useful! You should definitely learn this one.