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..
