Accomplishments are special tasks that your Virtual Pro must perform. Completing an accomplishment will upgrade one of your Virtual Pro's stats. You can view your Accomplishments at any time from the Main Menu -> VIRTUAL PRO -> ACCOMPLISHMENTS and also during a single-player game from the Pause Menu. As the description says, you need to complete 100 of these accomplishments for the achievement but this is not an as daunting task as you may think. There are many accomplishments that require almost nothing from you, i.e. Complete 4 ground passes (default controls ) in a match etc. I recommend targeting all the Arena, Beginner Shooting, Beginner Passing and Beginner Mental Accomplishments as these are all very simple and only require tasks that can be competed in 1-2 matches.
One more thing to note is that not all accomplishments can be unlocked on the easier difficulty settings or with two controllers. Make sure you check the header on every accomplishment page on the in-game accomplishment list to see what difficulty etc. you are allowed to play on in order to unlock the accomplishments on that particular page.