[Ruby] What does 'yield' do in plain english?

[Bob Marley]

Hello,

I am trying to follow some code that uses the keyword 'yield' but I am
getting confused what this keyword is actually doing:

require 'rubygems'
require 'mysql'

def with_db
    dbh = Mysql.real_connect('localhost', 'root', '',
'buildwatch_development')

    begin
        yield   dbh
    ensure
        dbh.close
    end
end

with_db do |db|
    res = db.query('select id, status, build_url from watches')
    res.each{|row| puts "#{row[0]} : #{row[1]} : #{row[2]}"}
    res.free
end

The explanation for yield from the programming ruby book 1st ed, for me was
equally confusing:

def fibUpTo(max)
  i1, i2 = 1, 1        # parallel assignment
  while i1 <= max
    yield i1
    i1, i2 = i2, i1+i2
  end
end
fibUpTo(1000) { |f| print f, " " }


I can kind of follow from the simple example:
def threeTimes
  yield
  yield
  yield
end

threeTimes { puts "Hello" }

My concept of yield is off as I interpret it as a kind of substitution, ie
the def threeTimes example. But in the first two example if you're going to
use the yield once, why bother?

Kinda lost,

miene