Okay kiddo, so you know how Poland is a country in Europe and how some other countries in Europe are part of the European Union, right? Well, a while ago, Poland became part of the European Union too. This means that Polish people can now move to other countries in the EU, like the UK or Germany, to work or study.
Because of this, lots of Polish people decided to move to other EU countries, especially the UK. They did this because they thought they might be able to find better jobs, make more money, or have a better life in general.
When they moved to other countries, they had to go through something called "migration." This means that they had to leave their homes in Poland and go live somewhere else. They had to pack up all their stuff, say goodbye to their family and friends, and start a new life in a new place.
Since the EU accession, lots of Polish people migrated to other countries. In fact, it's estimated that over one million Polish people moved to the UK alone! They brought their skills and knowledge with them and have helped make many other countries even better.
So, that's what migration from Poland since EU accession means. It's just Polish people moving to other countries in the EU to make a better life for themselves and their families.