These are the maximum upper bound and minimum lower bound values that it is possible for a value to take. For example the bounds for the roll of a die are 1 (lower bound) and 6 (upper bound). Not all variables have bounds: for example, when coin tossing the number of tails until you get a head is bounded below by 0 (you can't have a negative number of tails), but unbounded above, so it has a lower bound, but no upper bound.
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