This achievement is retroactive with Title Update #8, which means if you have completed 100 daily challenges prior to the title update, the achievement will pop once you start the game up.
If you haven't completed 100 daily challenges, you will have progress towards the achievement (I personally had 61/100 done when I booted the game.)
Daily challenges are (usually) simple challenges that require you to use weapons and equipment you normally wouldn't use (kill 5 spectres with the Wingman, kill 10 grunts with frag grenades, win 5 matches, etc.) You can have a maximum of 9 daily challenges at any given time. If you don't like a certain challenge you have, you can discard it by pressing
. Once you complete or discard a daily challenge, it clears out and replaces it with a new one for every challenge you've completed every 24 hours. Do 100 of these and the achievement is yours!