That is throughput and how is it calculated?

I am studying Little's Law and I am wondering how all the variables are acquired for these calculations in the context of project accelerated with Kanban, especially throughput.

Throughput in Kanban refers to the amount of work delivered over a certain period. No matter how many work items your team has in progress, this metric ignores anything unfinished.
Consider a workflow where throughput is calculated on a weekly basis (you can use any time range, but you should compare throughput of a team using the same range). In five weeks, this team delivers 5, 7, 3, 5 and 8 tasks respectively. Therefore, average throughput is calculated as (5 + 7 + 3 + 5 + 8)/5 = 5.6 or 6 tasks per week if we round up to the nearest whole number.
To manage such metric make sure you are tracking team's items that were done on a regular basis and decomposition approach is the same.
Such metric won't work for you in case you have tasks with dramatically different effort for work to be done. E.g. 1 week your team has committed to finish with 6 small items and managed 5 of them (throughout 5) and next week took 1 huge task and done with it (throughout 1).