Cadena in Spanish means chain, in tango it refers to any short sequence that is repeated, where the leader and follower go round each other and advance slightly.
Cadenas can involve a sacada by one (e.g. by the leader) followed by a similar sacada by the other (e.g. the follower), joined with a few steps involving pivots or turns. So, the whole effect is like a series of links of a chain. The word cadena in tango is used to indicate a concept, not a specific sequence. There are hundreds of different cadena variations.