I am trying to understand the code used to determine if a year is a leap year, I am stuck with this part

(year & 3 || !(year % 25) && year & 15 ? 28 : 29)

I don't get what year & 3 or year & 15 means. I know in some programming languages the ampersand sign is used for concetanation, I don't think that is the case here.

I know what && means, just not sure what a single ampersand on it own means.

I have read a bit about the leap year test regarding numbers being divisible by 4,100 and 400. But I can't find anything abohut & 3 and & 15.