What does the “Just” syntax mean in Haskell?

Question

I have scoured the internet for an actual explanation of what this keyword does. Every Haskell tutorial that I have looked at just starts using it randomly and never explain

Answer 1

Function if (cond :: Bool) then (ifTrue :: a) else (ifFalse :: a) must have the same type of ifTrue and ifFalse.

So, when we write then Nothing, we must use Maybe a type in else f

if balance < reserve
       then (Nothing :: Maybe nb)         -- same type
       else (Just newBalance :: Maybe nb) -- same type