This is somewhat contested in the trans voice community, but I believe it's at least an important topic to inform people on.
"Glottal attacks" are, very simply (please don't yell at me), when you enter into a vowel in a very (relatively) harsh manner. This is best displayed in the phrase "uh oh."
"Vocal fry" is, also very simply, when your voice rasps or growls because it's not given enough air support to produce a clear tone. This one is a little hard to demonstrate, but it's somewhere in between a whisper and your normal voice. You might be able to find it if you vocalize a tone, let's say the A in apple, and then slowly tone down how much air you give it until you're in a whisper. You probably went through a raspy fry in-between.
Mastering control of these two things are absolutely not necessary to get a cisgender passing voice, but they can help. They're a nice thing to focus on at the last stage of your voice training if you truly want to give yourself every advantage to be gendered correctly. Below is a set of three wordlists, with varying properties.
The top is a set of words that start with "H" which is relatively difficult to attack into or fry immediately after. At the bottom is a set of words that begin with a vowel, or an unvoiced consonant leading into a vowel, starting from a pause. These are relatively easy to attack into and fry after. In the middle is a combination of these two, where the "H" words exist to help dampen the word that follows to train you to avoid a glottal attack entering it or frying at the end for transfeminine voice training. For transmasculine voice training, the middle order can be reversed, leading with a word that is relatively easy to attack and fry into a word with which it is difficult to do either to train attacks and frying into speech.
Tresfeminine voice training should work from the top (the easiest for them) to the bottom (the hardest for them) and transmasculine voice training should do the opposite, also switching the order of the words in the middle section as described above.
Level 1: H word into a vowel
Honey
Hawaii
Hurried
Heroism
Harness
Level 2: H word into a vowel followed by an unvoiced consonant or pause into a vowel (reverse for masculinizing)
Transfeminine:
Harrowing Obstacle
Hapless Egg
Hastened Animal
Horrible Action
Hunted Official
Transmasculine:
Elderly Heron
Altered Horse
Torn Harness
Particular Holiday
Ornate Hairstyle
Level 3: Unvoiced consonant or pause into a vowel
Apple
Target
Oats
Pet
Eternal
Happy training!