I read it in an article and I don't understand the meaning of it. "not without some merit". From the context I guess the meaning is: there is some sense in it. However I don't find in the dictionary (vocabulary.com or thefreedictionary.com) an explanation that apt my understanding. Nor I've found an expression with "merit" that can explain it.
From your linked article:
Uber deployed all the conventional arguments, stating – not without some merit – that the mayor acted on behalf of the taxi industry and that Uber was good for minorities.
There's more here to be confused by than the expression, "not without some merit".
The expression is used parenthetically. So you should be able to remove it from the sentence completely and decode its main meanings: the mayor took the taxi industry's side. Uber is good.
Adding "not without some merit" shows that the author agrees with Uber, at least in part.
"Not without" is a double negative that is NOT used here for emphasis. It means, "It would be wrong to say this is completely false". That's not as strong as saying "it's completely true".
"not without some merit"
contains a figure of speech called litotes (pronounced lī′tə-tēz′, or lĭt′ə-, or lī-tō′tēz). Litotes is a backwards way saying something positive or negative. The logic of litotes is derived from the notion that two negatives make a positive. Or as @Mitch, above, puts it, litotes is "a rhetorical trope that makes a weak positive out of a double negative."
Instead of saying
_________ has very little merit,
you can say
_________ is not without merit.
Or, instead of the strictly negative
His nonsense knows no bounds,
with litotes, you could say with a mixture of positive and negative,
Of nonsense he has no small measure. [that is, as to nonsense he possesses the opposite of a small amount, which is a large measure]
Litotes can be a slippery concept in that it is somewhat difficult to define (i.e., your ability to define it is not without difficulty), but once you've "got it," used sparingly it can add a little spice to your style of writing and/or speaking.
Instead of saying
There was a huge crowd at the protest rally,
you could say with litotes
There was no small crowd at the protest rally.
Litotes adds an element of irony to your style, and irony when used in moderation creates in your audience a certain bond which sometimes cannot be forged without it!
Litotes is when an affirmative is expressed by the negative of the contrary (OED). The OP litotes - not without some merit - suggests that the speaker thinks that the thing under discussion has some merit.
The problem with litotes is that it is not always clear why you are choosing to express yourself using a double negative. Why say "not without some merit" rather than "this has merit"?
In fact litotes often implies a subtext which requires context to be understood. Used well, it can be used effectively for a variety of purposes. For example:
"...you didn't look completely out of place"
"...the fact you were drunk didn't make you useless"
to minimise impact;
"...don't worry, it wasn't the worst thing in the world"
to sound modest;
"...I'm not unfamiliar with Beethoven's music"
to highlight how preconceived notions may, in fact, be wrong;
"...this isn't a bad road"
But it is easy to use litotes clumsily and the result can make a speaker sound pompous, ridiculous and falsely modest.