Okay kiddo, so imagine you have a big family tree with lots of branches. Now, let's say you wanted to focus on just one part of your family - your mom's side, for example. You could zoom in on that part of the tree and kind of make a "mini tree" just for that side of the family.
Well, that's kind of what scientists do when they study organisms and how they're related to each other. They look at the big "tree of life" with all the different species and try to group them together based on how similar they are.
A subclade is like a smaller version of a bigger group. Let's say there was a big group called "primates" that included monkeys, apes, and humans. Scientists might then look specifically at the apes and say, "Okay, now let's make a smaller group just for the great apes - gorillas, orangutans, chimpanzees, and humans." That would be the subclade - a smaller group that's part of a larger group.
Hope that helps!