Team Able is one of the main teams of the Desert Rangers that are deployed often for completing various missions. Their leader is Captain Peppard. In 2102 when Team Echo was assigned the job of completing the late Ace's mission, Able and Team Charlie were tasked with stopping a range war in western Arizona.
You'll first hear about them when you ask General Vargas about being given the duty of completing Ace's mission. You'll meet them face-to-face for the first time outside Ranger Citadel after you save Highpool or Ag Center. After you leave the citadel, you'll hear them on radio for the first time.
Team Able is an obvious reference to the famous 1980s TV show "The A-Team". Peppard will also use the famous quote of A-Team's leader Hannibal when you first meet him. Especially, the name of all four Rangers of Able are actually the surnames of the real actors of the TV series.