Absorption rule in Boolean algebra
One way to grasp it would be with concrete examples.

So an expression like:

"is a man" OR "is a man with a sister" (i.e. A = is a man and B = has a sister)


In this expression, if you're a man, it doesn't really matter whether you have a sister or not, you are to be counted.

or perhaps closer to a program, in an answer to "which words should we select?":

"words that start with W" OR "words that start with W and end with S"


You can see that the second condition identifies at most a subset of the first and is therefore redundant (and can safely be lopped off).

In code, you would simplify an expression like:

if s.startswith("W") or (s.startswith("W") and s.endswith("S"))


to

if s.startswith("W")


This might be why it's handy to know as a computer scientist. Conditional expressions have a tendency to become complex (and therefore difficult to reason about). Being able to simplify them safely is often a good refactoring move.

Another way to grasp it would be visually, with a Venn diagram with A and B overlapping circles. In that case, A AND B would be their intersection (the overlapping part) and the expression A OR (A AND B) would mean all points inside A or inside the intersection. Since the intersection is by definition all within A, specifying the (A AND B) intersection as an alternative is redundant.