require uses the current directory that you are running the program from

require_relative uses the directory of where that program itself resides

For example, if a program is in ~/code and is called 1.rb and you've done a cd to that directory

cd ~/code

and you try and run the ruby program with

ruby 1.rb

then within 1.rb

require './2.rb'
require_relative '3.rb'

both will work.

However if you are in another directory, say

cd ~/tmp

and you try and run the program with

ruby ../code/1.rb

then you will get an error such as

/Users/chanweiyan/.rvm/rubies/ruby-2.4.2/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:59:in `require': cannot load such file -- ./2.rb (LoadError)
	from /Users/chanweiyan/.rvm/rubies/ruby-2.4.2/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:59:in `require'
	from ../code/1.rb:1:in `<main>'

when trying to use

require './2.rb'

whereas using

require_relative '3.rb'

still works ok _because the reference (to 3.rb) is relative to which directory the program (1.rb) is located in.

require_relative('path')

is the same as:

require(File.expand_path('path', File.dirname(__FILE__)))

require_relative complements the builtin method require by allowing you to load a file that is relative to the file containing the require_relative statement.

When you use require to load a file, you are usually accessing functionality that has been properly installed, and made accessible, in your system. require does not offer a good solution for loading files within the project’s code. This may be useful during a development phase, for accessing test data, or even for accessing files that are "locked" away inside a project, not intended for outside use.

For example, if you have unit test classes in the "test" directory, and data for them under the test "test/data" directory, then you might use a line like this in a test case:

require_relative "data/customer_data_1"

Since neither "test" nor "test/data" are likely to be in Ruby’s library path (and for good reason), a normal require won’t find them. require_relative is a good solution for this particular problem.

You may include or omit the extension (.rb or .so) of the file you are loading.

path must respond to to_str.

reference link: https://stackoverflow.com/questions/3672586/what-is-the-difference-between-require-relative-and-require-in-ruby