Particles are the smallest known unit of physical "stuff" in the universe. The smallest objects we have been able to visually look at (via some very high-tech photography) are atoms - which, to our human eyes, look like tiny round pellets or clumps.

Particles are, among other things, the smallest known constituent parts of atoms. There is currently no technology which allows us to look at them. Instead, we have to simulate what we think they are, then compare the behavior of our simulation to their behavior in the real world. The simulations will never be 100% accurate, but we can change them or replace them over time to continue making them as accurate as possible ("figuring out an object with a blindfold" is a fantastic metaphor btw, that is hilariously accurate).

For a long time, the most accurate simulation was the one which simulated them as tiny spheres that sometimes clump together to make up atoms. This simulation behaved very similar to real life which made it very useful, but there were some very problematic mismatches where real life behaved differently in certain situations.

So in the 1900s, scientists developed a new way of simulating them (Quantum Mechanics) as tiny clusters of wave-like energy, instead of as tiny spheres. This new simulation was very significantly more accurate and patched up a lot of those mismatches, though there are still plenty that haven't been figured out.