Lark notation is a way to reference a relative tree address, an alternative to using a numerical axis. It's valid syntax as part of a wing address.
The syntax alternates between using -/+ and </> for head/tail. It always starts with a -/+, then can follow with a </>, then back to -/+, and so on. For example, -<+>+ means "the tail of the tail of the tail of the head of the head of the subject" or, put another way, "descend into the head, then head, then tail, then tail, then tail."
Lark notation is usually only used for trivial cases, like +< or -.foo. It's generally bad practice to use long expressions like +<->+<-<+>->-. You should also use faces like p.foo rather than lark notation like -.foo where possible.
A couple of examples in the dojo:
:: get the 4th item in a cell::> =foo [1 2 3 4 5]> +>+<.foo4:: get the sample of the ++add gate::> +<:add[a=0 b=0]