Can you explain what it means?
Let's start with a stranger, Mr. X. If you go up to him and engage in conversational interaction, or he approaches you, and the interaction is sufficiently satisfying for you to exchange names (and if Japanese, meishi), then you have made an acquaintance. Your acquaintances are those to whom you can put a name or about whom you can state something, even if only in the form "Thingy from Solihull who sells billiard tables". Friendship is not required, though many people do indeed confuse the two, not least on Facebook (there should be a button for "(un)acquaintancing".