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A block is a chunk of code passed to a method. Two syntaxes:

# do/end — for multi-line or visual emphasis
[1, 2, 3].each do |n|
  puts n * 2
end

# braces {} — for one-liners (idiomatic)
[1, 2, 3].each { |n| puts n * 2 }

Most iteration in Ruby uses blocks. The receiver method yields to the block:

def three_times
  yield 1
  yield 2
  yield 3
end

three_times { |i| puts i }   # 1, 2, 3

yield calls whatever block was passed. Use block_given? to check whether one was provided:

def maybe_log
  yield "hi" if block_given?
end

Common block-taking methods:

[1, 2, 3].map { |n| n * 2 }       # [2, 4, 6]
[1, 2, 3].select { |n| n > 1 }    # [2, 3]
[1, 2, 3].reject { |n| n > 1 }    # [1]
[1, 2, 3].reduce(0) { |s, n| s + n }   # 6
[1, 2, 3].each_with_index { |n, i| puts "#{i}: #{n}" }

Most methods return the result of evaluating the block on each element — not just nil.