Tokens are a very simple and powerful way of creating little reusable calculations.
You can use tokens in your calculations to refer to other lines, not unlike how you refer to cells in a spreadsheet.
A token can refer to a specific line (ie. line 2), or relative offset (ie. the line above). The default is to refer to a specific line.
Using Tokens:
You can copy, paste and drag tokens around your document. Tokens that refer to a specific line will continue to point to that line even if it moves. Tokens that refer to a particular offset will always keep that offset.
If you hover your mouse over a token, the line that it refers to will be highlighted in blue or pink. The token's title will also indicate the line that it refers to. For example, 1L↑ means 1 line up.
Make a new line by pressing return, then type an operator like plus (+).
A token referring to the line above will be inserted for you.
Drag an answer from the answer column onto your expression.
Click "Insert Answer Token" in the toolbar to create a token that refers to the line above.
Using the numerous menu commands under the "Edit" menu.
For example, you can type command-\ to insert a token to the previous line, or command-1 to insert a relative token to line 1.
Double click the token to show the token editor.
You can also right/control click on the token and choose "Edit…", or select a token and then press "Return".
You can change the token's type (line or offset), and set its value.
Press return to close the token editor, or escape to cancel your changes.
Tips
You can type "line1" to quickly create a token that will refer to line1.