Diphthong means, very simply, two vowel sounds.
Example: Although the word "I" (as in "I am") is the vowel "I", is is audibly or phonetically sounded as:
Ah and ee = two phonetic vowel sounds.
Being able to identify diphthongs is crucial in singing because you need to know which vowels you are singing on, and whether they are GOOD VOWEL SOUNDS OR NAUGHTY VOWEL SOUNDS (read more here).
Exercise: When you are singing, listen out for these tricksy diphthongs:
EY EE as in "Maybe."
EYE EE as in "Bite."
EEE UH as in "Ear.'
UH OO as in "Bone."
O EE as in "Boy."
A OU as in "House."
AAH EE as in "Time."
So, when singing a diphthong, which vowel sound should you sing on? The first? The second? Both?
There is no hard and fast rule. You should probably try all the options and decide yourself but, my advice is usually that the first vowel sound is more open.
If this is the case then sing the first vowel sound and use the second vowel to tie up the note, like a full stop.
Time is sung TAAAAAAAAAAAH (eem)
I is sung AAAAAAAAAH (eem)
Bone is sung BOOOOOOOOOOH (oon)
Good luck and let me know how you get on!