{1..10}
From one end to the other, the shell lines up the sequence — with a step, if asked.
The sequence expression counts from x to y, up or down, and takes a step: {1..10..3} jumps three at a time, {a..z..2} walks the alphabet two by two.
Leading zeros on either end request padding: {01..10} comes out two digits wide throughout. The step and the padding arrived in bash 4.0 — in older shells the expression passes through unexpanded.
$ echo {1..5}
1 2 3 4 5
$ echo {a..i..2}
a c e g i
$ echo {01..10..3}
01 04 07 10