What does "plus colon" means in shell script expressions?

What does "plus colon" means in shell script expressions?

What does this mean:
if ${ac_cv_lib_lept_pixCreate+:} false; then :
$as_echo_n "(cached) " >&6
else
ac_check_lib_save_LIBS=$LIBS
Looks like ac_cv_lib_lept_pixCreate is some variable, so what does "+:" mean?
Where to learn complete syntax of curle brackets expressions? What is the
name of this syntax?