Why 1 plus 1 equals 1 in Boolean algebra?

In Boolean algebra, 1 + 1 is not "1 plus 1", it is "1 or 1". Since 1 is like true and 0 is like false, this is like asking "of the two values 1 and 1, is one of them a 1?", or asking "of the to values true and true, is one of them true?". Because there is at least one 1, the expression is 1 (true).

Other examples:

1 + 0 = 1 (There is at least one 1).

0 + 0 = 0 (There is no 1).