Dummy root for conjugations are فعل for triple roots, and فعلل for quadruple roots.

In the context of explaining grammatical rules, saying فاء الفعل / عين الفعل / لام الفعل (and more rarely لام الفعل الثانية) refers respectively to the first, second, and third (and fourth) letter of any given verb. So saying الفعل "أكل" مهموز الفاء means that this verb's first letter (figuratively "fa" but actually "a") is a hamza, and معتل العين means that the second letter is a vowel, and so on.

An old fashioned way to precise vowels when there's ambiguities, particularly in printed works where legibility is a problem, is to refer to each letter with its fa/3ain/lam equivalent. So أمِنَ بكسر الميم: اطمأن و لم يخف could be expressed as أمِنَ و تكتب مكسورة العين: أي اطمأن و لم يخف to convey that the second letter (even if it's not actually a "3ain") has the kasra diacritic.

On the other hand, فعل is more than just a dummy verb. It means "to do", and it's the verb to use for sentences like "what are you doing?" or "he did it" (where the nature of the act isn't the focus of the sentence or left ambiguous and unknown). Through derivations from the same root you have "act", "to react emotionally", "to react within itself" (like chemicals), "to make up"... Though not all theoretically possible cases of the derivation forms from فعل are valid words for common use.

But فعلل is a true dummy verb.

There's other dummy words like ones used for vowel balance in "vertical" poetry, like the تفاعيل in علم العروض. For example, the balance of بحر الوافر, one such pattern, follows this extra-long/long/short vowel layout مفاعلتن مفاعلتن فعولن and one such verse following this isبحور الشعر وافرها جميل. Or filler words used in conversations that don't actually mean much.