B Help with a string theory equation

What do you know about it? What have you tried? Please show an attempt.
That's the equation that Michio Kaku uses to illustrate string field theory. On the right hand side, the first half is what we call a kinetic term (for motion of the string) and the second half is a potential term (for interactions of the string).
Thank you so much

Urs Schreiber

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