A glottal stop is the thing you do when you say “uh oh”, specifically the short pause you do right before you say “oh”, if that makes sense.
Saying that “word-initial vowels are preceded by a glottal stop” is a technical way of saying that when you pronounce words that start with a vowel, you have to make that stop you do in the middle of “uh oh” before saying the word. Like, if you say “free installation”, you do a short pause to mark the beginning of the word “installation”, rather than pronouncing it like “freenstallation”.
Hope that clears it up.
Edit: I would like to add that this is not really a rule you should focus on, I think it will come quite naturally to you. Of course, just like in English, the normal Japanese use of the glottal stop is not as obvious as in the “uh oh” example, so don’t worry about it too much when you’re learning to speak.
I would add that it’s often very non-obvious (especially in songs), like when the word preceding を ends with a vowel, for instance in the phrase 日本語を勉強する, but a little clearer in 仕事をする and way clearer in スポーツをする.
But don’t worry about it, @Gamar779043 , it will come naturally