In other words, these are similar audiences. Facebook creates such audiences based on the original audience. For example, it can be an audience of website visitors, which we take from a pixel.
Next, we use algorithms that analyze the entire volume of data of the original audience. This is the behavior of people, which sites they look at, how old they are, what their gender is, what their interests are, how much time they spend online, etc. Facebook takes all this data, finds the most frequent matches in it and, based on these matches, looks for the most similar people among all users of the Facebook advertising network. The bigger the source audience, the better: more data for the algorithm to analyze and map. In general, lookalike opens up to the advertiser a wide range of possibilities for scaling campaigns at the expense of a new audience.