To understand the concept of 'structure' as discussed by Anthony Giddens, we need to delve into his theory of structuration. According to this theory, structure is not merely external or outside human action; instead, it plays a dual role in influencing human behavior.

1. Structure as 'constraining': Giddens contends that structures shape what people do by providing guidelines and scripts for social behavior. These structures include the norms, rules, and laws of society, which individuals must navigate in their daily lives. They set the limits within which individuals can act, thus constraining human behavior.

2. Structure as 'enabling': Despite being constraining, structures also facilitate actions. They offer the knowledge, tools, and resources needed by individuals to perform tasks and achieve goals. Structures enable individuals to act within certain contexts by providing a framework that can be drawn upon in various situations.

Therefore, according to Anthony Giddens, 'structure' in his theory of structuration is both constraining and enabling. This dual nature of structure allows it to shape human action while also being shaped by it in a continuous loop of interaction.

Hence, the correct option is: constraining and enabling.