Often I'll use (pp) or (message) to inspect a data structure in elisp. For example lets say I have a list l:
(setq l '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15))
(message l)
=> (1 2 3 4 5 6 7 8 9 10 11 12 ...)
Often times I want to see the entire list instead of the truncated version. That's where eval-expression-print-length comes in. Setting this variable to nil results in the entire list being printed.
(message "%s" eval-expression-print-length)
=> 12
(setq eval-expression-print-length nil)
(message "%s" l)
=> (1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)