Lets say you don't use a license for your open source project. And you don't put anything about a license anywhere in the code..is this okay to do?
Legally, in many (most?) jurisdictions you maintain copyright on what you create even if you do not specifically claim it.
As a result, if you haven't given permission to copy your code, people must legally assume that they do not have such permission. Putting a license with your code is merely a written statement outlining what you give permission to do with your code.
If you don't have a written statement like that, lawyers might get nervous because you could sue users of your code claiming that they didn't actually have permission to use it.
DISCLAIMER: I am not a lawyer, this is not legal advice.
Of course it's okay - it's your code, you can do whatever you want with it. However, if you don't specify a licence, the defaults defined by copyright law apply to your code. This may or may not be what you want. For instance, it usually means that your users cannot redistribute your code. That's a legitimate choice, but it's not what people expect when you call your application "open source".