This is the 278th most frequent Polish word.
sobą
"Sobą" means "oneself" or "with oneself" in English.
The word 'sobą' is used reflexively here, indicating that the action is directed back at oneself.
Martyna rozmawia sama ze sobą.
Martyna is talking to herself.
In this context, 'sobą' indicates accompaniment, meaning to bring something along with oneself.
Weź parasol ze sobą, bo może padać.
Take an umbrella with you because it might rain.
Here, 'sobą' refers to the subject directing attention to themselves, often used in a self-care context.