Lambdas are anonymous functions. You can define them and hold them in variables.

Here is how to define them:

lam = -> {puts "hello"}
lam.call # => hello

lam2 = -> (x){puts x}
lam2.call("frank") # => frank

If you define your lambda as below you can call it in more ways.

other_lam = lambda{puts "foo"}
other_lam.call # => foo
other_lam.()   # => foo
other_lam.===  # => foo
other_lam.[]   # => foo