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This is called an equation but I don't see an equals sign anywhere. Is it implicit somehow? Or am I missing it?
At top it should read “L = …” (as it does in the compact form printed on the coffee mug image).
Here, L represents Lagrangian density. The integral of L over spacetime (between some initial and some final time) is the action S, which therefore is a functional of the particular configuration the various fields take. The classical configuration of the fields (solutions of equations of motion) is the one that minimizes S between initial and final times. Quantum mechanically, the amplitude to find final field configuration given initial field configuration is obtained by integrating e^iS over all field configurations. Modulo a few details* the workings of the universe ensue.
*joke
Why is this equation presented in such an unformatted manner? Wouldn't it be better to align the terms to make patterns recognizable, rather than wrapping them like a sentence whenever the line is full?
Yup. That said it seems like a quick blog post from 2016.
There's been more comprehensive breakdowns since