'Standard Average European,' in phonology and phonetics is a term used to refer to any language in the Indo-European language family that is typically (Western)European. The term is often used to contrast English with that is considered 'normal' for an Indo-European language, which all share some features of phonology and grammar.
Even non-Indo-European languages like Hungarian, Finnish, and the famous case of Basque appear to be SAE on the surface. In fact, if you don't pay close attention to the words, you can easily mistake Basque for Spanish because they sound similar, even though Basque is an extremely different language. The similar phonology of these languages to SAE languages is due to centuries of proximity to them.
English, on the other hand, is a rogue Indo-European language, due to its relative isolation in Britain and America (And to a lesser extent, Australia) and the fact that it is truly a melting pot of languages. It is a phonetic nightmare to anyone who didn't grow up speaking it, and a challenge to phonologists. Some of the main differences between English and SAE are the letter 'r,' the plosives, the alveolars, and of course, the vowels, illustrated here:
- The letter 'r' in English is a very different from the 'r' of SAE languages. In English, this sound is technically a back vowel with bunching or retroflection. This is an extremely difficult sound for non-native speakers to make. SAE 'r's are usually alveolar trills or flaps, as in Spanish or Russian(an alveolar flap is basically the 't' sound in 'water.'), or a uvular trill or fricative like in French or German.
- The stops, or plosives, of SAE languages, are slightly different from those of English. In English, we aspirate voicless stops /p/, /t/, and /k/ (except after /s/), meaning we release the stop with a hard puff of air. This is one of the things that will give and English speaker a heavy accent in another language. Also, there is very weak voicing on /b/, /d/, and /g/. So, to say, a Spanish speaker, our 'b' sounds like their 'p,' and our 'p' just sounds crazy.
- Alveolar sounds (/d/, /t/, /s/, /z/, /l/, and /r/) are pronounced further back, at the alveolar ridge, in English than in SAE languages, where most of them are much more dental, that is, at the teeth of course.
And then of course, the big thing that makes English different from SAE languages are the vowels. Most of this is due to The Great Vowel Shift which happened shortly after the time of Geoffrey Chaucer. Basically, English vowels are one point higher (In Australia it can be two) than in SAE languages. This means that what we perceive as the sounds a, e, i, ('short') o, aw, and ow are actually /e/, /i/, /ay/, /o/, open /o/, and /aw/ in IPA and in SAE languages. English is the only language that would think of pronouncing 'a' like we do.
English vowels have an obscene amount of onglides and offglides, mostly on /e/ ('bait'= /beyth/) and /o/ ('mope' = /mowph/). This results in English not having 'pure' vowels (Actually, Russian and French often have complex vowels too). Most SAE languages only have about 5 or 6 phonemic vowels, whereas English has 16 (17 or 18 in some dialects). This is becuase English phonology makes differences between tense and lax and open and close vowels
So, next time you consider making fun of a European's accent, remember that English is the one that's messed up, not them. Or, you could just make fun of them anyway, cause they are European afterall.
Of course, in a broader linguistic sense, this term is a grammatical one. It was coined by linguists studying Native American languages, notably Hopi, to point out the stark differences between the syntax of these languages and the syntax which is ingrained in any SAE language. For instance, things we take for granted such as past, present, and future are technically non-existent in some languages. Unfortunately, my area of expertise is in phonetics, phonology and historical linguistics, and I do not know much about the field of grammar and syntax. If someone knows syntax well, please feel free to add on to this write-up.