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why is there this _ sin after every word?
Are you talking about the underscores that you see in variable and function names?
It's just a style of coding. It increases readability rather than jamming all the words together. We're used to spaces between words but we can't use spaces in variable names, etc... so we need a substitute for that.
Another style is to make the first word lowercase and every word after that uppercase. Again, to increase readability. There's no technical reason it has to be that way.