Micros (MES, MNQ, MCL, MGC and others) have the same price and tick size as their parent but a tenth of the tick-value. They make proper position-sizing possible for accounts under $25,000 and are what most prop-firm evaluations start traders on.
Commissions per contract are lower but higher as a share of profit, and liquidity is slightly thinner than the full-size contract.
Example: a trader with $5,000 risking 1% ($50) can trade one MES with a 4-point stop (16 ticks x $1.25 = $20 risk) with room to spare. One full ES with the same stop risks $200, or 4%.
Related: es, nq, tick-value, position-sizing