Chained missions are randomly chained missions (there is some vague logic behind them ...) but they are also persistent missions.
In layman's terms, they are a first step in having persistence in mission boards.
IIRC the first mission of a chain is like other missions (i.e. wiped after the 15 something minutes mission board refresh) but after you completed it, the "follow up" mission will be persistent for about a week. Meaning, for a few days, wherever you dock (and the faction giver is present), the mission will be available.
Not really a big deal, but a step in the right direction, if you ask me.