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Methods can take a block and yield to it — the magic that powers Ruby's iterators.

def twice
    yield
    yield
end

twice { puts "hi" }    # prints 'hi' twice

yield passes args to the block and returns the block's value:

def times_two
    yield 5
end

result = times_two { |n| n * 2 }   # 10

For named, reusable blocks, use Proc or Lambda:

double = ->(n) { n * 2 }   # lambda — strict on arity
double.call(5)              # 10
double.(5)                   # also works

Lambdas check argument count and return returns from the lambda. Procs are lenient and return returns from the enclosing method. For new code, prefer lambdas (stricter, less surprising).