When I was exploring the gambling board I saw multiple time this word that people were saying underdog playing. I search it for know more details. But I still confusion, Can anyone explain me it in example in both cases sports and slot games?
First, that doesn't happen to slots. The word "underdog" is used only in sports betting. Slots don't have that because they only rely on algorithms. We are the underdog there.

So, the explanation of underdog is betting for the unexpected team to win. That means they are expected to lose because they are weaker than their opponent. Now, there are a very low number of gamblers who bet on underdogs because they are a high-risk type of bet. Why? Because they are unexpected to win. This is where the spread will help but you will have to be wise about the choice.