Question

why the caps are put on head of i k j in vectors
?

Answer 1

'caps' refer to 'unit' vectors

Answer 2

Yeah, they're little hats. Also, think of vectors as kind of like objects. Like, a "chocolate ice cream" or "nuts" and then depending on how much of them you want, you multiply them by a number like 10*icecream+50nuts gives you a unique tasty dessert. Except you can't rotate nuts into icecream...

Answer 3

Any vector that has a magnitude of 1 is a unit vector. It is the most fundamental vector in a particular direction. They were given a special symbol, a hat, for their fundamentality.