For loop in R Programming Language is useful to iterate over the elements of a list, dataframe, vector, matrix, or any other object. It means, the for loop can be used to execute a group of statements repeatedly depending upon the number of elements in the object. It is an entry controlled loop, in this loop the test condition is tested first, then the body of the loop is executed, the loop body would not be executed if the test condition is false.

A for loop is a programming structure used in R to iterate over a sequence of elements, performing a specific set of operations for each element in the sequence. In a for loop, a variable is assigned to each element in the sequence, and the code block within the loop is executed for each value of the variable.

The basic syntax for a for loop in R is as follows: 

for (variable in sequence) {
  # code block
}

In this syntax, "variable" is a placeholder for the variable that will be assigned to each element in the "sequence", which is a vector, list, or other iterable object. The code block within the loop contains the specific set of operations to be performed for each value of the variable.

For example, the following code demonstrates a simple for loop in R that iterates over the values of a vector, calculates their squares, and prints the results: 

x <- c(1, 2, 3, 4, 5)

for (i in x) {
  squared <- i^2
  print(squared)
}

This code will output the following: 

[1] 1
[1] 4
[1] 9
[1] 16
[1] 25

In summary, a for loop is a powerful tool in R for automating repetitive tasks and iterating over sequences of elements. It can be used in a wide variety of applications, from data analysis and visualization to machine learning and statistical modeling.