you can already use it. Just download or clone the repository and copy the dist folder into /var/lib/grafana/plugins/ (e.g. /var/lib/grafana/plugins/carpetplot/dist). It should be built and working. If not, just use "npm install" and then "grunt" to build it.
The heatmap is based on a histogram (just take a look here: http://docs.grafana.org/features/panels/heatmap/, it's explained there very well). There is time in any resolution on x axis and values on y axis. Color (of a bucket) represents frequency of a specific value in a specific time. There is usually different number of buckets in every column.
In a carpet plot there is time on x axis but always in day resolution (one column = one day) and there is also time on y axis (from midnight to midnight) in a chosen resolution (hour => 24 buckets/day, 15 minutes => 96 buckets/day, minute => 1440 buckets/day). And it is easy to implement another resolution. There is always same number of buckets in every column. (With exception of first and last column, which depend on selected time period).
Bucket represents one value (or aggregation of multiple values that fall into the same bucket - same time and day) and the values are distinguished by a color.
This chart shows hourly gas consumption in a building. You can see that the biggest consumption is every day between 5 and 7 o'clock in the morning. And that the consumption has been very low since 19th May.
This is just one usage and there are many more possible - e.g. load of the server during a day, number of visitors, state of an alert - just showing 0 and 1, etc.
I hope this is at least a bit helpful.