Input the text you want to use in the top box and click the "Fragment!" button. Your fragmented text will appear in the bottom box and can be copy/pasted to a document for saving and printing. For a full description of the concepts and options, read this overview.
This tool takes a long text and fragments it into shorter, single-line "reads". Reads overlap, allowing for the full text to be reassembled.
Coverage refers to the number of times each nucleotide (here, letter) is seen by the sequencer. It's effectively replicates, higher coverage means you can be more confident in your results.
Indicies are unique sequences added to samples before sequencing so that multiple samples can be sequenced together without the data getting confused or misassigned.