Taking a look at the API’s source code on github, this API is just pulling the data from NOAA. Specifically, it is simply executing the FTP request: ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_trend_gl.txt. Taking a look at this file, the documentation is as follows:

Estimated CO2 Daily Global Seasonal Cycle and Trend (ppm).
The estimated daily global seasonal cycle and trend value for CO2 are determined from the daily averaged CO2 data from the four NOAA GML Baseline observatories. A smoothed seasonal cycle and a smoothed de-seasonalized trend curve are determined for each observatory record at daily intervals. An estimated global seasonal cycle and trend are computed by averaging the four individual observatory seasonal cycle and trend curves at each daily interval.
For details on the curve fitting process see
https://www.esrl.noaa.gov/gmd/ccgg/mbl/crvfit/crvfit.html
CO2 is expressed as a mole fraction in dry air, micromol/mol, abbreviated as ppm
NOTE: In general, the data presented for the last year are subject to change, depending on recalibration of the reference gas mixtures used, and other quality control procedures. Occasionally, earlier years may also be changed for the same reasons. Usually these changes are minor.

I have the same question. Can someone tell what are cycles and trends ?