You put the code on the internet for people to see. That's open source. If you want to make your project free software, you should also attach a free software license to your project that will allow other people to redistribute, modify and redistribute modified copies of your work.

Such license are:

• MIT: "I don't care what you do with my software, just use it"
• Apache: " Like MIT, but more legal terms thrown around"
• GPL-3.0: "You can do whatever you like with the software, but you must release your modified work under the GPL so other can benefit from it."

Hope that explains things a little bit. Don't hesitate to ask more 😄