Okay kiddo, let's talk about signal subspace! Imagine you have a really big picture with lots of different colors and shapes, and you want to focus on just one part of it. You can do this by looking at the "subspace" of the picture, which means you're only looking at a smaller part of it.
In the same way, when we talk about signal subspace, we're talking about a smaller part of a bigger signal. In science and math, we use something called linear algebra to study signals, which is like a big toolbox of ways to understand how signals work.
One of the things we can do with linear algebra is figure out which parts of a signal we care about the most, and which parts we can ignore. We call this the "signal subspace," because it's like a smaller part of the overall signal that we're paying attention to.
So, let's go back to our picture example. Imagine that you have a picture of a big flower garden, but you really want to focus on just the red flowers. You can create a "subspace" by only looking at the parts of the picture that have red in them, and ignoring the rest.
In signal processing, we do something similar. We focus on only the parts of the signal that are most important to us, and ignore the rest. This helps us understand how a signal is changing over time, or how different parts of a signal are connected to each other.
So, that's the basic idea of signal subspace - it's a way of focusing on certain parts of a signal to better understand how it works.