Hello Jbonn,
it seems to me that you are referring to some examples on I2C peripheral. Maybe from STM32L4 family? In order to work in slave mode, the user must enable at least one slave address. OwnAddress1 is part of the initialization struct and I2C_ADDRESS is literal(constant) defined using #define preprocessor it would be the i2c address you would like address with your slave device. If you are new to this topic I would recommend you to take a look on online training available here. In case I did not fully understand your question let me know.
Petr
