If you have given your fingerprints to authorities in an EU country, for example in Italy, Bulgaria, Hungary or Greece on your way or in other countries where you applied for asylum and don’t want to stay there but continued your journey, you might be threatened to be deported back to that country. This is based on the so-called Dublin-regulation which was signed by all EU member states as well as Iceland, Liechtenstein, Norway and Switzerland. The rule says: you have to apply for asylum in the first country where you arrive.
If you arrive in another country, the authorities will check first for your fingerprints, and, second, they will ask you the route you came through and, third, check the visa database about you. Fingerprints are taken and stored in the so called Eurodac-database.
When you are threatened with a “Dublin”-deportation, you can become active in order to struggle against it and that is via the courts on a legal level and also by getting organised. Sometimes, It takes a long time to overcome the fingerprints problem, but there are many people who succeeded in the end. Here you can find useful information on how the different countries deal with Dublin-returns. You can also find hints to strategies on how to stop a Dublin deportation.