Your content is yours: you own it, and can use it howsoever you wish. You may also license it to other people to use it.
By posting something here, you grant Stack Exchange a license to host and reproduce your content. Of course you do: otherwise they couldn’t legally do so. The license is non-exclusive, meaning that you can also license your content to other people (i.e., post it elsewhere). It’s still your content, and you are still the copyright holder.
To be specific, the licence is CC BY-SA 3.0, as specified in the footer of every page. The full license text is, of course, written in legalese, but it’s fairly straightforward. However, Creative Commons themselves provide a neat human-readable summary, as follows:
(Note, this is addressed to the consumer of the content, so the you here is the person reading and potentially reproducing the text. You yourself are “the licensor” — by posting your content to a Stack Exchange site, you are granting this license.)
You are free to:
- Share — copy and redistribute the material in any medium or format
- Adapt — remix, transform, and build upon the material
for any purpose, even commercially.
The licensor cannot revoke these freedoms as long as you follow the license terms.
Under the following terms:
Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.
No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits.