(additional personal note: being a square wave, in logic the zero volt signal will be indicated with 0, while the 3 volt signal will be indicated with 1)
This is relatively advanced, but you
will come across it sooner or later. There is a very important difference between digital signals and voltages.
In digital systems information is transmitted from the source to the receiver by
analogue signals. Usually those analogue signals are voltages, but they can be currents, light levels, frequencies (e.g. frequency shift keying), phase, radio waves, sounds (e.g. morse telegraph), etc. In all cases the reciever is an
analogue circuit that
interprets the analogue signal and - provided it meets specified criteria - treats it as a digital signal. Example receivers are logic gates, photodiodes+transimpedance amplifiers, radios, human ears. Example criteria include ranges of voltage levels (e.g. >V
ih <V
il), timing relationships (e.g. t
s, t
h), frequency ranges/combinations (e.g. DTMF), intervals....
...you get the general idea.
To come to your particular example "zero volt signal will be indicated with 0, while the 3 volt signal will be indicated with 1". Firstly, most logic
levels aren't 3V. Some are 0-0.4V/2.4-5V (TTL), others are 0-0.3V
cc/0.7V
cc-V
cc (many CMOS), and so in.
More importantly, some digital 1
signals are represented by
low voltages and 0
signals by high voltages. A classic example is chip select (CS) signals which are often "active low" signals shown on datasheet IC pinouts as /CS. Here the chip is selected when the voltage on that pin is low, e.g. <0.3V
cc which for a 3V supply would be 0-1V.
So, once you start to look at real circuits, you will need to understand those concepts.
You can also begin to see how logic analysers are used: they contain receivers, which immediately convert the voltages into digital signals. You specify the input ranges and active high/low,
and then forget about them - since all internal filtering, processing, display is done in terms of the digital signals.