What features of a word make a word sound natural in a language. For instance in two made up words 'mobify' sounds more natural in English than 'jlkrtz'.
By 'natural' you seem to be referring to what sounds (or phonemes) can be combined in what order. This is called phonotactics.
For example, mo in your example mobify is a combination of a consonant and a vowel that fairly often occurs in English, in words such as motor (for simplicity I'm ignoring here that spelling doesn't exactly reflect pronunciation - ultimately phonotactics is about what sounds/phonemes can combine and English spelling can be very misleading when it comes to pronunciation - but I hope the examples are straightforward).
But your example jlkrtz cannot be an English word because several combinations in this word are 'illegal' in the sense that they do not follow the sound laws of English. jl is not a legal consonant cluster, kr is, but krtz is not.
English does allow fairly long consonant clusters, such as in twelfths /twɛlfθs/ (four consonants in the coda). But consonant clusters of this length are rare and only a fraction of all possible combinations are allowed. Just try replacing /θ/ with /b/, /p/, /m/, /n/, /z/ or /k/, none of these are allowed.
For further details take a look at the 14 rules in this Wikipedia article.
P.S.: Nobody really pronounces very long consonant clusters except in very special contexts. twelfths has /twɛlfθs/ as so-called citation form. This pronunciation would be used when talking to learners of English (including children) or when speakers are aiming for very clear and accurate pronunciation. But in any common communication task at least one of the four coda consonants would usually be left out, even by newsreaders.
I think it depends of concrete language. For example in some languages are words with only consonants.
There is the phrase in (both) Czech an Slovak languages which worth of the article in Wikipedia: Strč prst skrz krk (translation to English, pronunciation, explanation in Wikipedia).
For Czechs and Slovaks it is natural phrase.
In Armenian language there are surnames like Mkrtychan. There are more people with this surname you can find in Google.
Every language has its own set of rules, and I think that exact answer on your question possible to find in Generative linguistics, not in phonology only. Why phrase Strč prst skrz krk is natural for Czech and Slovaks, but very bizarre for another people, for another human beings with similar speech organs?
There exists the Generative grammar of Chomsky, which explains construction of sentences. I think if Generative phonology would really exists like branch of science it can give proper answer to your question.