Vowel modification is the act of changing the vowel you are singing to better sing a note. Farther forward vowels tend to be easier to sing on higher notes, while farther back vowels tend to be easier on low notes. To feel this, sing one of your lowest notes on an "uh" vowel. While singing it, switch aggressively to an "ah." You may have difficulty even making the change, but if you do you'll notice the note is noticeably harder to sing. Similarly, if you sing a high note on "ah" and adjust to "uh" it will be noticeably more difficult.
So let's say you need to sing the words "I'm in luck" up near the top of your range. What you'll actually want to sing is something more similar to "Ah'm ehn lahck." The closure and the end of I'm (the ee sound) makes the higher note hard, similarly the "ih" sound is too far back. Finally, the "uh" in luck would become an "ah" almost like the word lock.
To figure out how to modify any given sound, try to recognize where in your mouth you say a vowel. For example, you can feel an "ih" sound in the roof of the back of your mouth. Attempt to move the vowel forward in your mouth without changing any other aspect of it. It will become an extremely "eh" version of "ih" but not fully "eh." That is the form of modification we want. If you can find the feeling of singing forward and back effectively enough, you won't even need to think about saying the word "wrong" but will instead simply sing "I'm in luck" in the front of your mouth. It will still come out in the modified way, but will be much easier mentally.
Vowel modification is tricky if you don't already have good control over the shape and vowels your mouth produces or have the ear to recognize slight vowel changes. If you need extra help with vowel modification, sign up for a lesson with me or leave a comment and I'll try to help out.