Micro is a RTS term for how well you control your units. In starcraft for example you have the unit "stalker", which can teleport. Professional players with very good micro can use that teleport to always teleport out a Stalker just before it would die. This means because of good micro they don't lose their units.

The opposite is macro, which mainly talks about how good a player is in managing the game on a grander scheme like managing their resources and having an overview over the whole map

(I hope my example is still accurate it has been years, that I watched an rts game)