ELI5 - What is empathy and how does one feel it?

I’m not sure what empathy is or how to feel it. It’s sometimes left friends and partners feeling frustrated with me when I can’t comfort them in the way they need and it causes me to be upset that I don’t understand it. I want to understand what it’s like.

Edit: tagged as chemistry because I guess technically it’s brain chemistry.

Edit: I’m talking about this issue with my therapist later today.

Edit: just got done with therapy. Turns out I do feel empathy, but it just comes off as not caring because I get frustrated that I can’t always figure out how someone needs to be comforted. I might look into getting tested for autism because it happens a lot.