The thumb(T) is used to support your right hand by resting it on the bass pickup as shown in the photograph.Most modern bass guitars have two pickups.One close to the fretboard and a second one nearer the bridge.In the early stages of playing,it may be easier to place your thumb on the front pickup as the string tension is not quite as"hard" as on the back pickup.The tone will also sound fuller and heavier,and it will also be easier for your right hand fingers to play the strings.
You should also experiment by playing with your thumb resting on the back pickup as the sound is clearer and has more attack and"bite".