My computer is moving like a turtle

I'm having a problem with my computer at home. Lately, it's been functioning really slow. I can't figure out why and what happened. My brother told me maybe because of overloaded saved files and applications in it. Is that true? Please let me know what steps might be done for this. Thanks in advance.

Use tune up utilities software. It will increase your system's speed by optimizing it.

What are utilities software? I'm sorry but I'm not familiar with them.