Being "all-in" can mean different things to different people. However, an obvious, first-impression of that phrase is that a) you are serious about getting to know that person and possibly building a relationship with them, b) do not want to talk to other prospects (even if you aren't yet sure or ready to be in an exclusive relationship with the one you are talking to), and c) not into playing around.
Personally, saying you are "all-in" is a very bold statement and shows real initiative coming from a man, in my perspective at least. It often seems that some men want to move at the speed of a sloth when getting to know a woman and while that may be a defense mechanism, it isn't truly loving with the love of Christ, in my opinion, because it's essentially saying that you want to spend a pro-longed period of time gradually getting to know someone (who is one of many) instead of just taking a chance and seeing if the two of you are compatible--which is most easily accomplished by talking to one woman at a tine. Basic compatibility can be determined pretty quickly and easily in my opinion and if it is determined to be there, it doesn't mean a man has to feel pressured to be in a serious "let's talk about marriage" relationship with that woman but moreso just focus solely on her, getting to know her, her getting to know him, spending as much time interacting as possible, and then eventually, when both are ready decide the proverbial "where is this relationship going?" they can have that discussion and decide if they want to call it quits or proceed with dating. Most women are not ready to get married tomorrow and I think many men have the misconception that they are. I personally want to spend a great deal of time getting to know someone before we seriously even consider having that type of discussion; however, if a man moves at the speed of a sloth then honestly I get bored because it's hard to read that kind of behavior and often comes across as an unspoken way of telling me that he is not really very interested. I never talk to only one man at a time but if a man, after an extended period of time, was to say he was "all in" and wanted to focus on me solely, then I'd be more inclined just to focus on him solely as well. Afterall, fair is fair.
In my personal opinion, if a man is unsure about a woman it means either one of two things to me--a) he's really not that into her (meaning he thinks he can do better, is more focused on someone else, not very attracted to her, etc.) and/or b) he's unsure of what he wants or his readiness to begin the process that could lead to being in a relationship. Either way, honesty is always the best policy in my book and if a man wants to be "all-in" then he needs to say it. Equally, if he's not into LDRs, not physically attracted to me, not ready for anything too serious--again, he just needs to say it. Any man who did this would not upset me but gain my utmost respect and I'd be happy to pass him on to any of my interested and available single girlfriends because being brutally honest shows leadership and initiative--two VERY SEXY traits in a man.
As women, we so often desire to be there for a man that we are interested in--even if he is super busy. I know I do. I want to support him in all his activities that he has going on, encourage him, and be there for him in any other way possible that I can. Sometimes this may mean just having to give him some downtown or space, but again, this is something he needs to communicate. As far as women desiring to have a man in their lives, even if they are happily single, financially independent, etc. this is imbedded in our DNA. Afterall, Genesis says God made Eve for Adam so that she could be his helpmate. So, being a nurturing, supporting friend, girlfriend, fiance, wife, etc. is part of what makes us women. I just wish so many more men realized that and appreciated it instead of seeing it as something to be afraid of.