English words with vowel initial tend to get a glottal stop. This occurs in most dialects, so a native speaker wouldn’t notice its presence or absence; they will just hear it as a “normal” vowel.
Formally speaking, a glottal + vowel is perceived as an allophone of that vowel alone.
However, there is a significant difference between pronunciation of word-initial vowels at the beginning of an utterance and in its middle. Words that begin with vowels get a preceding glottal stop only if they are at the start of an utterance. Compare apple and the apple (the rhymes with bee). There will be no glottal stop in the second case, and it is very easy to notice.
In other languages (like Hawa'iian), the presence or absence of the glottal stop (including the initial position) changes the meaning. One example is ʻai (with the initial glottal stop). It means food, and one should not use its counterpart word with a plain vowel at the beginning.