Hello; I apologise for any typing errors, as I am typing from a handheld device. However, there is something that I do not understand about the standard model. I have read that this equation is a 'lagrangian' but I do not know what this means or how to work with one. According to what I have read, this equation describes how different particles interact with each other. My question is "how?"... How does this equation tell us this? Thanks.