Okay, so you know how there are different sounds in music that come from different notes? Sometimes those notes are close together and sometimes they are far apart.
A diminished third is when two notes are played together and they are closer than a regular third, but not as close as a second. It's like when you're walking and you take a small step instead of a big step, but you don't take a tiny step like a baby.
Sometimes musicians use a diminished third to make music sound more interesting. It can make you feel like something unexpected is happening, like when you see a bird flying backwards! But you need to use it carefully, like when you want to add a little bit of surprise to a song.