Co znaczy imieniny(is it diffrent from birthday?) ?

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Yes, it's something completely different from birthday. It's a custom of celebrating a day of the year that's associated with one's given name (therefore "imieniny" can be translated into English as "name day"). The celebration is very similar to birthday and in Poland - in my opinion based on personal observation - celebrating the name day (imieniny) is even more popular than celebrating birthday. Similarly to birthday it is celebrated only once a year. If there are more than just one occurence of a name during the year people usually celebrate their name day on the day that comes as first after their birthday. For example the days associated to my name are in February, July, August and September. Since I was born in November, therefore the first occurence of a name day is in February and that's my name day.

A person whose name day is celebrated is called the same way as a person whose birthday is celebrated - solenizant (m) / solenizantka (f).

You should also pay attention to the fact that both "imieniny"/"urodziny" although in many cases refer to a singular celebration of one's name day or birthday act as plural nouns.

For a complete list of name days in Poland you can have a look at this website:
http://www.kalbi.pl/kalendarz-imienin