method definition in ruby, colon vs equals

method_def_ruby

$ irb
2.4.3 :001 > def foo(a:1, b:2)
2.4.3 :002?>   print "#{a}#{b}"
2.4.3 :003?>   end
 => :foo 
2.4.3 :004 > def foobar(a:1, b:2)
2.4.3 :005?>   foo(a,b)
2.4.3 :006?>   end
 => :foobar 
2.4.3 :007 > foobar(3,4)
ArgumentError: wrong number of arguments (given 2, expected 0)
	from (irb):4:in `foobar'
	from (irb):7
	from /home/samgooi4189/.rvm/rubies/ruby-2.4.3/bin/irb:11:in `<main>'
2.4.3 :008 > 
2.4.3 :009 >   
2.4.3 :010 >   
2.4.3 :011 >   def hello(a=1, b=2)
2.4.3 :012?>   print "#{a}#{b}"
2.4.3 :013?>   end
 => :hello 
2.4.3 :014 > def helloworld(a=1, b=2)
2.4.3 :015?>   hello(a,b)
2.4.3 :016?>   end
 => :helloworld 
2.4.3 :017 > helloworld(3,4)
34 => nil 

2.4.3 :016 > def hello(a=1, b=2)
2.4.3 :017?> print "#{a}#{b}"
2.4.3 :018?> end
=> :hello
2.4.3 :019 > def helloworld(a=1, b=2)
2.4.3 :020?> hello(a, b)
2.4.3 :021?> end
=> :helloworld
2.4.3 :022 > helloworld(3, b:4)
3{:b=>4} => nil