It is the basic standing asana in most forms of yoga with feet together and hands at the sides of the body. There is some contention between different styles of yoga regarding the details of the asana which results in some variations.
Although tadasana is a very basic asana, it is the basis and starting point for many standing asanas. As such, tadasana is important in the context of other asanas because it allows the body and consciousness to integrate the experience the preceding asana and prepare for the next. Further, as a common and fundamental asana, tadasana can help develop habits for further practice and asanas.
Asanas that help prepare for tadasana include Adho Mukha Svanasana and Uttanasana. Urdhva Hastasana is a very similar asana with the hands raised above the head.