No matter what language you speak, you know about a wide variety of completely unwritten rules that apply specifically both to your language and your particular dialect. Many of these rules deal with grammatical structures that require you to form your sentences with certain word orders to be meaningful, others refer to how your pronounce certain words, when to voice a sound and when to leave it voiceless. Sequential constraints in phonology refer specifically to the clusters of sounds that you are allowed to use when making words happen.
In English, consider if you are given the sounds /b/ /l/ /ɪ/1 and /k/. These four sounds can be arranged in a variety of ways that can form words that sound English: /blɪk/, /klɪb/, /bɪlk/, and /kɪlb/. Of these, only "bilk" is a real word, but they all sound like they are real, and, since we're dealing with word-formation from phonetics point of view, this is all we care about. Your mental phonetic constraint is what tells your brain that /kbɪl/ and /lbɪk/ just aren't words that could possibly exist in English. English phonology tells our brains that words cannot start with /kb/ or /lb/ sounds. Specifically, your brain knows that a word that starts with a stop consonant cannot be followed by another stop consonant (/kbɪl/), and if a word starts with a /l/ or /r/ sound, the next sound must be a vowel, not a stop (/lbɪl/).
In English, you are limited to three-sound consonant clusters at the beginning of words or syllables:
First Sound Second Sound Third Sound
----------- ------------ -----------
s p l
Cluster Example Word Transcription
------- ------------ -------------
spl Split /splɪt/
spr Spring /spriŋ/
spy Spew /spju/
str Strength /strɛŋkθ/
skl Sclaff /sklæf/
skr Screed /skrid/
skw Square /skwɛr/
sky Skua /'skjuə/
* An exception to these rules, these clusters are not permissible in English initial formations.
Imagine you're given the /kspl/ sound cluster. It is obviously not a permissible cluster to begin a word with, but can exist inside of words just fine, as in "explicit" /ɛksplɪsɪt/. In multisyllabic words, larger clusters are permissible because of syllable boundaries: /ɛk $ splɪ $ sɪt/. Note that the /kspl/ is broken by syllables into a syllable-final /k/ followed by a syllable-initial /spl/ as in our chart above. In the same way that syllable boundaries make "explicit" a permissible /kspl/ cluster, we know that "condstluct" /kandstləkt/ is not a permissible word in English, because it either has an /stl/ or /tl/ syllable-initial sound cluster.
Advertising in English often makes use of these sequential constraints to create new words in English that do not have meaning. Consider Bic, Xerox, and Spam, which were not in the lexicon when they were initially introduced, but because they follow the restraints placed on English words, they sound like real English words. This is also why we're likely never to have a company come out named Zhpleet /ʒplit/ because it breaks our cluster rules. For more information on this concept, see lexical gap.
1. The /ɪ/ sound is the vowel sound of "ship" and "lift." See International Phonetic Alphabet for more information on IPA transcription.