What is mortgage?

A mortgage is a long term loan taken to make a major purchase - like a house.

I don't know the word mortgage.
You can look up the meaning of a word in something called a dictionary: Dictionary and Thesaurus - Merriam-Webster Online

A mortgage is a document that gives a lender a security interest in your property. When you take out a loan you sign a mortgage note, which is a promise to pay back the loan. You also sign a mortgage, which says that if you don't pay back the loan the lender gets to take your property, sell it, and apply the money from the sale to the amount that you owe.