This is R basics (and, in some cases, general programming basics); you can find the answers to these questions in any introduction to R. In fact, you will learn this better if you investigate it a bit yourself rather than just asking. However, if you do need the answers:

• %in% is an operator that returns TRUE if the thing on the left appears anywhere in the thing on the right (i.e., if sume number appears in a list of possible numbers).
• value is some list of list of numbers. So `x %in% value` returns TRUE if x is in the list, FALSE otherwise
• ! means 'not'. So it changes true into false, or false into true. Basic boolean operator.
• "erase
  1 votes 1 thanks

I'll add a couple of comments.

1) Using R, it's easy to look up the meaning of functions:

?"%in%"

?"!"

for most functions you don't need the quotes:

?mean

2) The function is a little unusual in its form.  More typically, the function would be written as:

erase

I agree; I had never seen this kind of in-place replacement function in R before reading this question. I don't quite see why you would want to use it; I'm pretty sure R was designed with specific reasons not to do in-place changes to things.

I'm learning data science with R and in a book this examples were provided in function section. After reading this type of functions, I was wondering how it is actually working as it was little bit surprise for me. I have never seen this kind of function and even in stack overflow i haven't found this type of function. 

Thanks everyone for your reply

let say  vector  x

The %in% function returns true and false values.

Try:

Sal = c(99, 100, 101, 102)

Sal %in% 99

Sal[c(TRUE, FALSE, TRUE, FALSE)]

Same as:

Sal[c(1,3)]