A "Truth Table" is a means of checking that you have considered the effects of all the possible ways in which switches can be set, and that no dangerous connections will be made. The usual way of denoting that a switch is "ON" is to use the number "1", and when it is "OFF" to use the number "0".
The Truth Table in Fig. 7-12 describes the outputs you get when the switches of that example circuit (which happens to be the one we talk about later, and show in Fig. 7-14) are set in all possible ways. You can check that all possibilities have been covered by considering that if each switch can be either ON (1) or OFF (0), two ways, then there must be 2 x 2 = 4 ways of setting two switches together, 2 x 2 x 2 = 8 ways for three switches, and so on.
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