What does the Ruby method 'to_sym' do?

Expanding with useful details on the accepted answer by @cHao:
to_sym is used when the original string variable needs to be converted to a symbol.

In some cases, you can avoid to_sym, by creating the variable as symbol, not string in the first place. For example:

my_str1 = 'foo'
my_str2 = 'bar baz'

my_sym1 = my_str1.to_sym
my_sym2 = my_str2.to_sym

# Best:
my_sym1 = :foo
my_sym2 = :'bar baz'

or

array_of_strings = %w[foo bar baz]
array_of_symbols = array_of_strings.map(&:to_sym)

# Better:
array_of_symbols = %w[foo bar baz].map(&:to_sym)

# Best
array_of_symbols = %i[foo bar baz]

SEE ALSO:

When to use symbols instead of strings in Ruby?
When not to use to_sym in Ruby?
Best way to convert strings to symbols in hash
uppercase %I - Interpolated Array of symbols, separated by whitespace

to_sym converts a string to a symbol. For example, "a".to_sym becomes :a.

It's not specific to Rails; vanilla Ruby has it as well.

It looks like in some versions of Ruby, a symbol could be converted to and from a Fixnum as well. But irb from Ruby 1.9.2-p0, from ruby-lang.org, doesn't allow that unless you add your own to_sym method to Fixnum. I'm not sure whether Rails does that, but it doesn't seem very useful in any case.