Building at a much smaller than standard minifigure scale. Official sets at micro-scale are often labelled "mini".
A style of building that is smaller, often much smaller, than regular minifigure scale. Official sets at micro-scale are often labelled "mini", not to be confused with the scale used at LEGO® Minilands, which is larger than minifigure scale.
For example, a normal LEGO house is made out of dozens of bricks and features doors and windows about the size of a minifigure. At the small end, a micro-scale house may be only a single 2x4 brick with two sloped bricks on top of it at the small end. At the large end, it is the scale seen in some of the LEGO Architecture models, with doors standing at most one brick tall.
Canonical examples of micro-scale are the LEGO Creator "Mini" series, the LEGO Star Wars Advent Calendar, and the LEGO Architecture theme. lego-games often use micro-scale building, in combination with microfigures for player pieces.