Hi @JoanGalarza!
A metadata ("mapping") file is for providing additional context and information to per-sample reads (or individual features) --- that is usually some kind of data very specific to your particular study. For example, QIIME 2 sample metadata might include elapsed time, pH, elevation, surgery status, sample type, age, or body site. Check out the Metadata in QIIME 2 tutorial for suggestions on getting started with metadata. As well, take a peek at the Moving Pictures tutorial, which has a great example of a Sample Metadata file.
As far as importing your demultiplexed reads goes, please check out the Importing data section on Fastq Manifest formats - if your reads are already demultiplexed (per-sample), you can use this import type to get your reads into QIIME 2. Keep us posted and let us know if you have any questions! 