'This' is singular (one thing), 'these' is plural (more than one thing). However, we usually say 'that' when one thing is not nearby and 'those' when more than one thing is not nearby. Unfortunately, the meaning of 'nearby' can vary...

This is singular (refers to one single object), these is plural (refers to multiple objects).

This is my thumb. These are my fingers. Here is my wallet. These are bills. This one is a one-dollar bill. These others are five-dollar bills. In this picture, this is my wife, and these are my three children. This one is a boy, and these two are girls. In this car, this is the rear-view mirror and these are the side mirrors.

"This" is a singular pronoun that refers to a singular noun that is nearby in space or time. For example, if you are holding a book in your hand, you might say "this book" to refer to the book you are holding. "These" is a plural pronoun that refers to more than one noun that is nearby in space or time. For example, if you are holding three books in your hand, you might say "these books" to refer to the books you are holding. So, the main difference between "this" and "these" is that "this" refers to one thing and "these" refers to multiple things.

"This" is singular. "These" is plural.