Consider a simple production function:
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Y = zF(K,L)
Output = Total Factor Productivity * Function of (Capital, Labor)
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Think of TFP as all the environmental variables that make up "technological dynamism" or the economy's technological infrastructure (horse -> steam -> coal -> oil, human capital/education, etc.) that increase productivity in fundamental but intangible ways, and that can only be calculated indirectly.
If you hold TFP (z) and Capital (K) constant, an increase in Labor (L) (total number of hours worked) should increase output.
If investment in capital is decreasing (which it currently is) and subsequently the stock of capital decreases; or, if the TFP is decreasing (which is currently is), an increased amount of labor is required to keep output constant.
Weak/declining output = slow/negative GDP growth = "bad economy"
The "Cobb-Douglas" output model takes this simple model one step further by embedding the output elasticities of labor and capital into the production function, allowing one to build in diminishing marginal returns. I.E. every hour of labor or unit of capital above a certain point produces less output than the previous hour/unit. [1]
[1] https://en.wikipedia.org/wiki/Cobb%E2%80%93Douglas_productio...