What are open vowels? Explain with examples.

To really understand the answer to this question, you will need to pronounce some sounds and pay attention to how they feel in your throat and mouth. (Are you in a private space and not, say, in the middle of the dining hall or a quiet library? Perfect! Let us continue.)
Vowels and consonants are the building blocks of speech sounds. Vowels are considered the nucleus of a syllable.
Vowels (A, E, I, O, U), by their very definition, are produced by an open use of the vocal tract. Go ahead, make the sound "a," as in the word "father." Notice how your mouth opens wide on that first syllable?
Now, try another word that has a different vowel: the letter "u." Say the word "tube." Do you notice how your mouth makes a completely different shape as you say it? It is not like the open, tall-mouthed feeling you have when you say the word "father," is it?
Nouns might all involve an open use of the vocal tract, but they are not all the same. What is an open vowel (also referred to as a low vowel)? In short, it is a vowel sound that you make with your tongue positioned as far away as possible from the roof of your mouth. The term "low vowel" is sometimes used, as I said, and "low" refers to the low position of the tongue in your mouth.
Examples: "ARM." "GOT." Say these words aloud and notice how your mouth is open "tall" as you produce the vowel sound.
In contrast, if your mouth is almost closed when you produce a vowel, you are pronouncing a "closed vowel."
Examples: "SEE." "TOO." Do you notice how your mouth barely opens to make these sounds?
If the vowel is somewhere in the middle, with your mouth open midway, that is considered a "mid vowel."
Examples: "CUP." "AGO." You open your mouth a little bit but not a lot.
Linguistics professors often use a chart to illustrate the different positions the tongue takes as we pronounce vowels. However, I personally find it a little abstract. I am attaching a different vowel space diagram that actually shows a cross-section of the human head and the positioning of the tongue in the mouth when pronouncing different kinds of vowels.
Of course, you can also just say a given word aloud and ask yourself whether it has an open, closed, or mid vowel.
https://notendur.hi.is/peturk/KENNSLA/02/TOP/VowelSpace.html