"Few microns" means a very small measurement or distance. Microns are units used to measure extremely tiny things, so when we say "few microns," it indicates something being on a minuscule scale. Full definition
The extra downwelling IR (Infrared radiation) from extra CO2 does warm up the Knudsen layer of the ocean (the topmost few nanometres where all the evaporative action takes place) and perhaps few microns below it. (newclimatemodel.com)
For the energy required to come from the air the IR has to warm the air first but in fact the IR warms the topmost few microns of water first. (newclimatemodel.com)
This forms because heat is radiated from the top few microns net upwards — this layer is cooler than the underlying water because the radiation losses are quicker than mixing processes. (judithcurry.com)