Stock and flow is an economics concept referring to static value (stock) and transactions over a period of time (flow). This is a useful metaphor for producing content on the internet.
Flow is like the ephemeral feed of content we create on social media and forums. Stock is the evergreen content we create like blog posts and essays. Flow spreads quickly (viral sharing), but is short lived (pages out of the feed in hours). Stock spreads slowly (Google search), but grows steadily over time.