A FASTQ file is the raw output from the sequencer: it contains all your DNA reads, in the form of hundreds of millions of short sequences of letters (A, C, G, T), with a quality score for each letter.
It is the most comprehensive format because it does not depend on a reference genome: your data can be reanalysed from this point using a more recent reference or one released in the future. Bioinformaticians and researchers use it to align DNA reads and identify variants from scratch.
It cannot be read directly to discover your traits or genetic susceptibility: it needs to be processed using specialist tools. Indicative requirements: Linux or macOS, command-line skills, at least 16 GB of RAM (32 GB recommended) and more than 200 GB of storage.