Registers in Vim let you run actions or commands on text stored within them. To access a register, you type "a before a command, where a is the name of a register.
If you want to copy the current line into register k, you can type
"kyy
Or you can append to a register by using a capital letter
"Kyy
You can then move through the document and paste it elsewhere using
"kp
To paste from system clipboard on Linux
"+p
To paste from system clipboard on Windows (or from "mouse highlight" clipboard on Linux)
"*p
To access all currently defined registers type
:reg