PIE was the language (or languages) the proto-indo-European peoples spoke during their processes of migration into Europe, the Middle East and South Asia (around 6-4k years ago).

So most of the languages from those regions, from Hindi, to Persian, to Greek and English are all PIE, as these are all descendants from the PIE peoples.

We can reconstruct it by analysing these modern languages, their recent ancestors etc. and compare them.