'Yeísmo' refers to the convention in most dialects of Spanish (except Castillian and older Latin American dialects]) to pronounce the digraph 'll' as a palatal approximant or /j/, which is the 'y' sound in English 'yet'. This is as opposed to the /λ/ sound which it represents in a few dialects (basically like the 'lli' in 'million'). Apparently, around the Seventeenth or Eighteenth Century 'll' was pronounced palatal lateral /λ/, and 'y' was pronounced palatal approximant /j/. Southern Spaniards (most of the ones who settled in the Americas) began to confuse the two sounds, both becoming /j/. This is why most of the newer (17th Century on) cities of Latin America, as well as coastal cities with much contact with Andalucia have a yeísmo and not the older lleísmo.

A person or dialect with yeísmo is described as yeísta (the opposite is lleísta). Due to the yeísmo, a there is no distinction between valla/vaya, callo/cayo and llanto/yanto. This can, of course, cause confusion among native-speakers as well as non-native speakers. It's really a lot like how pronunciations of English can differ from Britain to America to Australia. An interesting point is that if you speak to a British person in Spanish, chances are he or she has a lleísmo because the standard learned over there is the Castillian dialect, whereas in America the standard Latin American yeísmo/yeísta dialect is taught.