Stock Watch: Very new to trading. I'm wondering what the nature of expiry dates.

When I made the purchases one the options was to set an expiry date, either good for one day or good through the month. I chose "good until the end of the month" and no further date was permitted.

Do my stocks expire then? Do I have to rebuy at whatever price it's at a month from now? Sorry about the green horn question but I haven't found an answer on Google.

Thanks!