@dkuku It’s really easy to understand it.

The best way is by practice, so let’s create an example code:

defmodule Example do
  def sample(bool) do
    with true = bool do
      :ok
    else
      _ -> :error
    end
  end

  def sample2(bool) do
    with true <- bool do
      :ok
    else
      _ -> :error
    end
  end
end

Now let’s use our Example module:

iex> Example.sample(true)
:ok
iex> Example.sample(false)
** (MatchError) no match of right hand side value: false
    iex:3: Example.sample/1
iex> Example.sample2(true)
:ok
iex> Example.sample2(false)
:error

If expression matches pattern then in both cases we have same results. More interesting is when match would fail. For match operator (=) Elixir raises an error and in second case we got :error (result of else clause).

Summary:

1. = and <- are not equal
2. = it’s a match operator like in any other place in code
3. a <- b is valid Elixir code which is often used in macros/special forms
4. <- unlike = would not fail when there is no match - instead else if defined is used - otherwise returns nil

Finally to understand it better we should take a look at documentation. Both of them are not part of Kernel, so firstly we should take a look at Kernel.SpecialForms.

Here are the links:

1. match operator
2. with/1 special form

As @logicmason said above (a long time ago) the right arrow (aka stabby arrow) -> is also used for case and cond (more here in the guide).

You didn’t provide more details but I bet that in your case something0 is either a case or a cond. It might also be a rescue (and even more rarely maybe a receive or an after).

But in all of these cases (except rescue) you’ll have a do (like case something do or cond do), right?

Consider the do as giving “something” to the block as if it was an argument..
So what’s happening in all of these use cases is actually a pattern-match between what’s given in and what’s on the left hand side of the arrow.

For the case it will be the value of the expression (it might be a function’s return or simply a variable or even a literal value). For the cond you can consider that the literal true is given..

So now you can pattern match what you want on the left of the arrow against what’s given in. In the case of a literal you can have a pinned variable on the left for example.

In any case you can consider this right arrow as a pattern-matching.

Now regarding the _ (underscore), it’s simply an equivalent of a wildcard (or an I don't care value) something that will always evaluate the pattern match as a positive match. For the cond the equivalent is true which is used as the last clause as you can see on the guide.

So now to answer your question simply,

something0
_ ->
  something1

could means that regardless of the value to what something0 is evaluated, execute (or evaluate or return) something1

Hope it makes sense..

-> is also used inside case and rescue clauses.