When viewing your online friends' statuses, you'll see something like below:
Level 10 Assassin Mastering 1-7
What does the "Mastering 1-7" mean?
The "Mastering" you refer to refers to two things.
Let's say we have a "Mastering X-Y" whatever on someone's XboxLive or Steam status..
X will refer to what Playthrough that player is on. Whether it be, 1, 2, or 2.5. Y, on the other hand, will refer to what quest of the Main Storyline that player is on. Since there are multiple quests that correspond with each playthrough (including sidequests mind you), these quests are also labelled with numbers.
In your case, "Mastering 1-7" Would refer to Playthrough 1, Hunting the Firehawk.