This (determiner)

1. Used to identify a specific person, thing, or situation.
2. Referring to the present thing, situation, or idea.
3. Used to introduce a following noun phrase.

Origin:

Old english thīs, of germanic origin; related to dutch deze and german dies.

Examples:

1. This book is mine.
2. What do you think of this restaurant?
3. This house is very old.
4. I want this shirt and those shoes.
5. This is what I was talking about earlier.