Learn in the textbookBinary Numbers and Radix
DefinitionA binary number (2進数) represents values using only two digits, 0 and 1, and each time you move one digit to the left, the size that a 1 in that digit stands for (its weight) becomes twice as large.
ExplanationThe number of different digits that one position can use is called the radix (基数), and the radix of binary is 2. Weights in the integer part double from the right as 1, 2, 4, 8, while weights to the right of the point halve as 0.5, 0.25, 0.125. Negative numbers are usually written in two's complement (2の補数), formed by flipping every 0 and 1 of the positive number and then adding 1.
AnalogyIt is like a row of lamps that can only be on or off, where the pattern of which lamps are lit stands for a number.
Memory hookThink of "base 2" as "once two pile up, move on to the next place." In base 10 (decimal), you move on only when ten pile up.