What it means? [id^=btn]:checked + ul {display: block;}

I saw a line of code in youtube video: [id^=btn]:checked + ul {display: block;} I understand it affects to all btn IDs (btn, btn-1, btn-2). But how? When I changed ID from "btn-1" to "btn1" it doesnt help. Im new programmer, have never seen [id^=btn] before. Where can I research it, can you help me? Because, when googling, it shows only Bootstrap, but there was no link belonging to bootstrap in HTML head. Thank you for your help!