To double means to add an amount equal to what you already have. An example: If you have one bottle of coke, and you get one more bottle, you have two bottles of coke. Two is the double of one, because $1+1=2$, which means you have doubled the amount of coke you had.
Imagine that you have four bars of chocolate, and someone gives you four more. You go from having four bars of chocolate to having eight of them. Eight is four two times, because $4+4=8$, which means you have doubled the amount of chocolate bars you have.
Think About This
What is the double of: 1, 2, 3, 4, 5, 6, 7 and 8?
$1$ twice is $1+1=2$
$5$ twice is $5+5=10$
$2$ twice is $2+2=4$
$6$ twice is $6+6=12$
$3$ twice is $3+3=6$
$7$ twice is $7+7=14$
$4$ twice is $4+4=8$
$8$ twice is $8+8=16$
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