A regular ability is one of the one or two abilities a Pokemon can have from a normal wild encounter or in-game trade. A hidden ability is a rarer third ability, introduced in Generation V, that usually only appears from special encounters like Friend Safari, Dynamax Adventures, raids, or Mystery Gift events.
Step 1
On Pokepedia, every ability page at /pokemon-abilities/levitate splits the Pokemon roster into a "Regular ability" block and a "Hidden ability" block. A Pokemon listed under Regular has that ability as one of its normal slots.
Step 2
Pokemon listed under Hidden can only obtain that ability through the generation-specific hidden ability encounter for their game. Some Pokemon appear in both sections for different abilities. The lists are mutually exclusive per ability.
Step 3
To verify, read the effect description at the top of the ability page. The effect applies the same way whether the ability slot is regular or hidden. The distinction is only about how the Pokemon can acquire it in-game.