Follow up question : Would it not depend on the time period ?

When reading Nicholas Riasanovsky's History of Russia it seems that Poland, along with Lithuania, was a powerful state in Eastern Europe between the 13th and 16th century. Enough to be a threat to the Great Principality of Moscow and be an important actor during the crisis with Boris Godunov.

So, it was not that weak at this time period right ? Compared to their Eastern neighbours ?

A good book on the subject is Poland: A History. I'm only answering since no on else did, so I will do so generally: for much of its history it had weak central authority and geographically powerful neighbors. The former was for a variety of reasons but ultimately due to a strong nobility. For a more general view on how political balance affects the state, I would suggest Fukuyama's The Origins of Political Order - he speaks of Poland's failed oligarchy but moreover about the balance of parliament versus a monarch, etc. The powerful neighbors for their part would consistently meddle in the country's (and Commonwealth's) internal affairs and war was nearly constant. Its presence as a buffer state with overlapping cultures made it a constant bargaining chip in the European "balance of power" until after the French Revolution.

It's not exactly correct. Throughout entire middle ages Poland was rather strong nation (mostly in X and XI century) which could successfully oppose any of the neighbouring nations. And at least from XV century to the 2nd of XVII century Poland not only could oppose other European powers but it was just one of them. Poland led successfull wars with Sweden, Teutonic Order, Russia.