The freeroll can be seen as a bonus. After all, you can play for free, so you can win risk-free money. It may be unclear for novice poker players how to participate in a freeroll poker tournament. It may happen that you receive an invitation via the website of the poker room where you are registered or that you receive a bonus code from an external website.
Access (+playing credit) to freeroll poker tournaments is usually provided as a bonus. For example, most poker rooms offer a freeroll bonus when you sign up new or when you make a reload deposit. It also happens that poker tournaments are played daily and you can participate in them without any conditions.