What does the ~> symbol mean in a bundler Gemfile?

You usually use this to tell bundler that it's ok to install some minor updates (last digit specified can vary) but not to install some major update.

SO

~> 2.0.3 means >= 2.0.3 ∩ < 2.1

and

~> 2.1 means >= 2.1 ∩ < 3.0

Read more at https://bundler.io/gemfile.html

From the bundler website:

The specifier ~> has a special meaning, best shown by example:
'~> 2.0.3' is identical to '>= 2.0.3' and '< 2.1.'
'~> 2.1'     is identical to '>= 2.1'    and '< 3.0'.
'~> 2.2.beta' will match prerelease versions like '2.2.beta.12'.

See https://bundler.io/gemfile.html and http://guides.rubygems.org/patterns/#pessimistic-version-constraint