why does my microsoft word has symbols at four corners like _| at the top left, |_ at the top right?

To symbolize monopoly; it covers all four corners of the playing field.

Just my guess.

It's the printable area/margins on your document. Everything in the document should be within those lines/markers other wise it might be cut off when you print it. There are other printers/software that can support bleed/printing with smaller margins.