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 pickup.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 string.
The left Hand
Left Hand Placement
your finger should be on their tips and placed just behind the frets(not on top of them)