What are twin primes?

Numbers which have only two factors (are perfectly divisible by only two numbers) i.e. one and itself.

For example,

5 is a prime number as 5 has only two factors, 1 and 5.

Twin-primes:
Two prime numbers whose difference is two are called twin-primes.
Two consecutive odd prime numbers are called twin-primes.

For example,
5, 7 are twin primes as they are prime numbers and their difference($7-5=2$) is two.