Support explaining how this code works

Question

HI everyone, 
 Could you help explain me how below code works? The character "_" is new to me and I cannot find any documentation for it. 
 apply(
 contains(
 index(
 local!childTask,
 "id",
 null
 ),
 _
 ),
 index(
 local!parent,
 "id",
 null
 )
 )

Kumar Agniwesh · Answer

first thing first, Apply function has been replaced with foreach. Or I would say Foreach is better way of implementing looping logics. 
 In your case, - would be replaced with value of the id from parent variable. You can implement same logic with ) a!forEach(
 items: index(
 local!parent,
 "id",
 null
 ),
 expression: contains(
 index(
 local!childTask,
 "id",
 null
 ),
 fv!item
 ),
)

Konduru Chaitanya · Answer

Hello mollyn126 
 apply() evaluates the given rule or function for all the items in the given list. Where in the apply the 'function' parameter is similar to the 'Expression' that you write in the a!forEach() but would only be a function here. 
 The '_' defines the item in the array against which the function would be evaluated. We use '_' to iterate all the items in the given list in the 2nd parameter of the apply(). 
 Also, Appian recommends to use the a!forEach() instead of apply() for better null handling. 
 
 docs.appian.com/.../fnc_looping_apply.html