After learning the Heian kata, Tekki Shodan is usually the next kata a student of Shotokan karate is taught. There are three Tekki kata in total - Tekki Shodan, Tekki Nidan and Tekki Sandan. Originally the Tekki kata were named Naihanchi, but Sensei Gichin Funakoshi renamed them. The name Tekki can be interpreted as "Iron Knight", "Steel Knight" or "Steel Horse Riding". The stance used in the Tekki kata is kibadachi, horse-riding stance, so this is rather logical.
The origins of the Tekki kata are uncertain, but they are apparently older than the Heian kata. According to one account, Tekki Shodan was created by Sensei Matsumura Sokon and Tekki Nidan and Sandan by Sensei Itosu. Apparently the Tekki kata were the first ones Sensei Funakoshi learned, and it took him ten years to do so.
Tekki Shodan is usually required when grading from 4th kyu to 3rd kyu. The other Tekki kata aren't usually required in gradings, so there is perhaps less emphasis on teaching them.