Apply 
f@@expr or Apply[f,expr]
replaces the head of expr by f.
f@@@expr or Apply[f,expr,{1}]
replaces heads at level 1 of expr by f.
Apply[f,expr,levelspec]
replaces heads in parts of expr specified by levelspec.
Details and Options

- Apply uses standard level specifications:
-
n levels 1 through n Infinity levels 1 through Infinity {n} level n only {n1,n2} levels n1 through n2 - The default value for levelspec in Apply is {0}.
- f@@@expr is equivalent to Apply[f,expr,{1}].
- A positive level n consists of all parts of expr specified by n indices.
- A negative level -n consists of all parts of expr with depth n.
- Level -1 consists of numbers, symbols, and other objects that do not have subparts.
- Level 0 corresponds to the whole expression.
- Apply always effectively constructs a complete new expression and then evaluates it.
- Apply operates on SparseArray objects just as it would on the corresponding ordinary lists.
- Apply operates on values only in Association objects.
- Apply[f][expr] is equivalent to Apply[f,expr].
Examples
open allclose allBasic Examples (7)
Apply gets rid of a level of lists:
Use the operator form of Apply:
Applying f to an Association keeps only values:
Applying a List to an Association is equivalent to Values:
Scope (10)
Generalizations & Extensions (2)
Applications (4)
Properties & Relations (5)
Leaves are visited before roots:
Total does effectively the same thing as applying Plus to a list:
Using ## in a pure function has the same effect as using Apply:
Three ways to apply a function at level 1:
Ordinary function application takes the list as a single argument:
Apply takes the elements of the list as separate arguments:
Text
Wolfram Research (1988), Apply, Wolfram Language function, https://reference.wolfram.com/language/ref/Apply.html (updated 2014).
CMS
Wolfram Language. 1988. "Apply." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2014. https://reference.wolfram.com/language/ref/Apply.html.
APA
Wolfram Language. (1988). Apply. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/Apply.html