The Facts on Ceceo, the Castilian "Lisp"
A student of Spanish has likely experienced Iberian Spanish dialects, such as Castellano, in which the /s/ sound in certain words of new world dialects has a "th" sound. The International Phonetic Alphabet (IPA) symbol for "th" is /Θ/, called either theta or simply eth. The Spanish ceceo/seceo, or practice of using eth, is a phonologically distinct practice having to do with vowels, and is not, as one may have heard, the command of a king who once spoke with a lisp.
The rumor goes back to a piece from López de Ayala remarking that that Pedro of Castile spoke with a slight lisp (ceceo), even though Pedro was king in the 14th century, and we have now found out that ceceo didn't enter the language as a phoneme until the 16th century. The use of Pedro's reported lisp should have entered the language through what we call prestige borrowing (talking like the upper class to sound upper class). However a lack of evidence has led researchers finally to dismiss the story as just a legend, and let de Ayala's remark be what it is.
We still deal with prestige dialect issues today, a stark example of which being the ongoing debate over ebonics, a proposed label for a large group of phonological traits peculiar to the African-American poor, which has been used against its original purpose in order to stigmatize African-Americans in the face of more prestigious dialects, like the "standard American" that is actually from Michigan.
Anyhow, ceceo is actually a valuable and natural trait that has evolved over time to suit the needs of Castellano Spanish. A very simple rule governing a lot of ceceo's behavior when sounds of modern Castillian are concerned is that, when a sibilant sound (briefly, /s/) comes before an /i/ or and /e/, the sound turns into eth. There were a lot of other sounds shared with Celtic, Provencal, Arabic and other neighbor languages in early Spanish that have now died out. It would take a lot of work to demonstrate all of them, and would not be particularly useful to a student not invested in phonology, but we will say briefly that the footprints of these old sounds, such as the voiced "zh" of French and Persian, are found in the distinctions now made by ceceo.
Ceceo serves to distinguish words that would sound the same if ceceo weren't applied. For instance, the reader may be an enthusiast of Cazadores Tequila. Cazador means hunter, and the sentiment is almost identical to that of the "master hunter" drink Jägermeister of Germany. A very simple example of the use of ceceo is to consider that casa means house and caza means hunt. Without ceceo, such as in the dialects of the new world, one would have to rely on context in order to make the distinction. Naturally there is no /i/ or /e/ in caza, so we know that the eth sound in caza must have replaced a much older root that orders its use.
it is good practice to try to use ceceo in one's Spanish when speaking to Spaniards, even if the student was not trained to use it. One will find that, in situations using ceceo, it actually can make pronouncing the word significantly easier. The /i/ and /e/ distinction ordering use of ceceo is a physical reality: /i/ and /e/ are tense front vowels, /i/ also being high, and eth is easier to deploy in this environment, since it doesn't require the speaker to tensely atriculate the tongue for an /s/, but rather to simply put it in the front of the mouth, in the way of the air that will deliver the vowel sound.
Try it! Take a word like decir. The /d/ of Castellano is already very soft, employing the tip of the tongue tapped just enough against the alveolar ridge to make the sound. Without moving the tongue after relaxing the /d/ articulation, eth can be pronounced in the same position: decir.