What exactly are singing keys?

I recently started singing at karaoke contests locally and people say
im really good at singing in key e.

I went online and did different karaoke songs and looked for what key
it was and noticed that im good at the ones that say Key D, Key E and
Key F. I can also do the ones that are Key A-C however not so well. I
also tried one that said Key G and could not even do it.

What exactly is Key D, E, F, etc?

I am singing country if that helps?

Some people have asked if I have an album or ever sang in a band
because they found me talented. So now I am thinking perhaps I need to
pursue this as my new hobby and take professional lessons if other
perceive as being good. Ive sung in my home for over 10 years but
don't know anything about music.