Gemination, sweetie, is when two consonants that sound similar are pronounced as one longer consonant sound. It's like a consonant getting a twin, but they grow together so they only count as one!
For example, imagine saying the word "button" really slowly. You would normally say "buh" "t'n" with a little pause in between. But with gemination, we would say "butt'n" without the pause, making it sound like one longer consonant.
People usually use gemination to emphasize certain sounds or to make the word easier to say. It can happen naturally in some languages, or people might do it on purpose for different reasons.
So, in summary, gemination is when two consonants grow together as one longer consonant sound. Try it out, and see how it feels to say words differently!