Can someone explain what is RNAV transition and what is the difference between RNAV transition and RNAV STAR. Because sometimes both types of procedures are published for an airport and I don't know what is the diference between those both types.
Your Question: "Can someone explain what is RNAV transition and what is the difference between RNAV transition and RNAV STAR. Because sometimes "both types" of procedures are published for an airport and I don't know what is the diference between those both types." (emphasis is mine)
On an RNAV STAR the Transition Fix (there could be several for the same STAR) is the beginning of the transition from enroute to the terminal arrival portion of the procedure, see (SEGMENTS OF A SID/STAR) in the Aeronautical Information Manual, as noted by @randomhead in the comments to the question.
At certain airports/terminal areas, because of the complexity, volume of different routings, and for clarity purposes the "Transition" routings and "Arrival" routings are published separately. At many locations both are published on the same page (or front and back of the same page).
Below is an example of an RNAV Star where both the "Transition" and "Arrival" parts of the STAR are published (separately).
The first image is the clearance a specific aircraft was given into KLAX (from FlightAware data). Note the routing into KLAX is GABBL (an RNAV Transition fix) to HLYWD (an RNAV fix where the Arrival portion begins).
The next image is the Transition showing the GABBL Transition RNAV fix
The next image is the Arrival beginning at HLYWD and ending at SEAVU.
An RNAV STAR will have one or more transitions that you have to file over to begin flying the STAR. The CHPPR ONE STAR to KATL has four transitions: BBABE, LEMKE, MTHEW, and RUTTH. When flight planning into KATL from the northwest (provided your aircraft qualifies per the notes) you would choose one of the transition fixes to begin flying the STAR.
Please also note that two of the four transitions are only for use by departures off of specific airports. BBABE and LEMKE are only used by departures off of KHSV and KCHA (respectively).