3D printing is an additive manufacturing technique, meaning that your part will be printed layer by layer. These layers have a thickness. Layer thickness in 3D printing is a measure of the layer height of each successive addition of material in the additive manufacturing or 3D printing process in which layers are stacked. It is one of the essential technical characteristics of every 3D printer; the layer height is essentially the vertical resolution of the z-axis. When manufacturing a project using additive manufacturing you deal with three different dimensions: X, Y and Z axis.
To understand what is a layer thickness it would be helpful to fully understand what it does for your 3D prints. Selective Laser Sintering is an Additive Manufacturing method that uses a powder bed fusion process to build 3D parts.
Powdered polymer build material, typically nylon, is transferred from containers (called the ‘powder delivery system’ in the picture) holding fresh powder onto the build stage in the process chamber with a recoating tool.
A laser then selectively scans the thin layer of powder, sintering together powder particles in the shape of the cross-section of the first layer of the 3D part. The build platform then descends one layer and the recoater transfers more fresh powder from the hopper to the surface of the first layer. Just like the first layer, the second cross-section of the 3D model is scanned and sintered. The laser scanning process simultaneously generates the current layer and adheres it to the previous layer, making a solid part.