What is registry error fix?

recently i downloaded and installed tcp optimizer, the one that speeds up your download speed. but since iam not very good in technical terms, i deleted it for fear it might "slow" my laptop, if unproperly set up. i also heard from pals that when you delete installed softwares, it might cause registry error. now my conecern is this guys, how do i perform registry error fix? what is the software to use to properly fix registry errors? can i do that without downloading softwares? please help. thank you all so much.