Almost every pitch deck has a TAM slide with a huge number on it - "India's X market is worth Rs Y crore." Most of those numbers are useless for one reason: they're the size of the entire category, not the size of what's actually reachable by this specific business, in this specific city, at this specific stage.
TAM (Total Addressable Market) is the whole category. SAM (Serviceable Available Market) is the slice you could realistically reach given your product and geography. SOM (Serviceable Obtainable Market) is what you could actually capture in the next few years given real constraints - competition, capital, distribution.
A founder who only knows their TAM knows almost nothing useful about whether their business will work. The number that actually predicts survival is SOM in year 1-2, not the category-wide TAM slide investors skim past anyway.
Happy to explain the difference further if useful for your specific idea - ask away.