In a twitter poll asking what the asterisk indicated, only ~55% of the respondents (total 338 votes) answered correctly.

This is an image of the left kidney obtained from the posterior aspect (during kidney biopsy). The asterisk indicates left pleural effusion. While the rib shadows are obscuring the view a little bit, we can clearly see that the anechoic region is above the diaphragm and hence not ascites. Moreover, you can see a beating structure (heart) adjacent to it. This should not be confused with right pleural effusion even if probe position illustration is not provided because liver is a big organ, and you would expect it to appear more prominent than the spleen in this image. It is not pericardial effusion as the anechoic area is continuous with the costophrenic recess. Below is the correlation with a normal CT scan. Always remember, POCUS sense = anatomy + common sense!

More images from the same patient: