Micro lots let small accounts follow proper position-sizing rules. They were the reason forex became accessible to retail traders with a few hundred dollars, which is also the reason many of those accounts were over-leveraged.
See standard-lot for the full size ladder.
Example: 3 micro lots of GBP/USD with a 30-pip stop risks 3 x $0.10 x 30 = $9.
Related: standard-lot, pip, lot, leverage