ok well if you know how to count then you're probably aware that every number you normally count to is made up of combinations of the same old set of single digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Notice there are 10 of them. It is a counting system based on 10 unique digits in other words, or in other words, it's called a base 10 system.
Your computer works on a base 2 system. That is, it only has 2 unique digits, 0, and 1. This however makes no difference where counting, or any other kind of numerical trickery is concerned. Any kind of math you can do with a system of 10 digits can be done with a system of 2 digits. For example if you want to count in base 2, or binary, you say 0, 1, 10, 11, 100, 101, 110, 111, and so on. Which is exactly the same thing as saying 0, 1, 2, 3, 4, 5, 6, 7, and so on. And number you can express in base 10 can also be expressed in base 2.
so why does a computer bother with base 2 instead of doing everything in base 10, or base 3 or whatever? well for one thing it makes designing the actual circuits of the computer very convenient. If all you have is 0 and 1, then you can decide to represent 0 by the presence of a low voltage in a wire, and 1 by the presence of a high voltage
From there you proceed to acquire some transisters. Like millions and millions and millions of transistors.
So you take a few transistors and wire them up into a clever little circuit with 2 input wires that does nothing but, for example, output a high voltage if both its input wires have high voltages on them, and otherwise outputs a low voltage. And you call this thing an AND gate, which is one kind of a logic gate.
So you make millions and millions of these AND logic gates. And while you're at it, you make millions and millions of OR gates, which are circuits that output high voltage, or "on", or 1, when either of its two inputs are on, or if both of them are on. And you make XOR gates, which output on, if either of its two inputs are on, but not if both are on. And NOT gates, which output on if their single inputs are off, and vice versa. And you can combine NOT gates with AND gates to make NAND gates, which output the negation of an AND gate. And so on.
these logic gate circuits are the basic building blocks from which actual computer circuity can be constructed. For example you can take a few logic gates and cleverly wire them together to produce a circuit that can store a value input to it, either a 0 or 1, and output the value on request. this is called a flip flop, and it's the basic unit of computer ram memory. all your computer's ram consists of millions and millions of little flip flop circuits, each one storing a single bit. It also consists of some other circuits for addressing all these flip flops of course, writing to them, reading to them. and you can also use logic gates to construct the various circuits that make up the cpu, aritmetic logic units, command processor, the works.
in your typical computer like the one you're using these computers tend to work with bits in groups of 8, called bytes. each byte consists of 8 bits, and it happens that an 8 bit binary number can hold any decimal value from 0 to 255
now an important thing to undersand about your computer is, that all these circuits you're building have no real conception of what numbers are. They're just flipping switches around, turning wires on and off. whether the bytes involved represent numbers, or codes standing for letters, or any other kind of data, depends entirely on the series of instructions being executed by the cpu (using those circuits made out of logic gates). whoever programmed the instructions decided what the data being worked on stands for in other words. none of it means anything to the computer itself, it's just a bunch of on and off signals.
so you see these same circuits can represent numbers, or letters, or colors, or anything else that you can represent with a set of numeric codes.