This achievement unlocks when you successfully complete 50 online races. You can view how many you've completed at any time by going to your stats screen and looking under Performance Summary for the completed online races number which will be in yellow. "Successfully" completing an online race translates to actually crossing the finish line in time after the winner does and obviously not retiring due to a disqualification or a failure.
You can boost this by finding a boosting partner and then continuously completing one-lap races on the Knockhill Tri-Oval track using the car class of your choice. Who wins doesn't matter as long as both drivers complete the race. The only settings you need to worry about are the one that keeps the lobby public and the one that keeps the minimum competitive rank on. Credit goes to TA user "R3DH4MM3R".