What does it mean if a girl asks if I'm single?

The same question means different things depending on the last few messages. If the conversation has been warm and she's been asking you things, the question is probably interest with a practical check attached. If the conversation has been thin and she's suddenly asking about your status, she may have heard something from a mutual friend, or she may be deciding whether to keep talking at all. Look at how many questions she's asked you, how long her messages are, and whether she's been the one keeping things going. A girl who is leaning in will usually phrase it casually and follow up. A girl who is hedging will often ask it and then go quiet, which tells you the question was a gate, not an opening.

If you're not sure, the honest read is that she wants to know where you stand before she spends more energy, and that's a reasonable thing to want.

The reply that fits this moment has three parts. The first is a plain answer to her question, given in a line or less. The second is a small acknowledgement that it's a fair thing to ask, so she doesn't feel silly for wondering. The third is a question or remark that moves the conversation somewhere she can take up, usually about what she's doing, what she's into, or something you're looking forward to. The answer should be honest and unembellished. If you're single, say so without a list of caveats. If you're not, don't use the question to hint at something complicated, because that is the message that creates the most trouble later. The question back should be light enough that she can answer it in one line, and it should not be a question about her ex or her history. You are trying to shift the topic to something you both can build on, not to start an interview.

After you answer, give it a normal amount of time. In most cases that means a few hours if you're both active during the day, or until the next morning if you're not. Don't follow up with a second message on the same topic, and don't send a clarifying line about your status. If she replies warmly, keep the thread going with ordinary conversation and let the status fade into the background. If she replies with something short, take the cue and match her length. If there's no reply at all after a day, leave it. A second text asking if she got your message reads as pressure. If the thread dies, it was probably her checking and deciding not to go further, and that's useful information. The move after that is a new conversation with someone else, not a chase.