What is PHP header? Explain to me like I was a 5-year old!

I'm wondering about what headers are. I use PHP strictly for HTML completion and I thought I had to send text/html header when the output was text and then image/jpeg header from a separate script which was used as source in an image tag but then someone suggested me to take away the headers because they made nothing... And I did and everything was the same as before... This made me wonder, what are headers, when are they used, both from an HTML perspective but also from some (?) other perspective and why could I remove mine?THANKS EVERYONE, I'M ENLIGHTENED :)
 
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