A straight is a hand made of five consecutive cards that are not all of the same suit.
This hand falls somewhere in the middle in terms of overall hand strength, just above a three-of-a-kind hand and below a flush.
To give you a few simple examples of a straight, the highest possible straight in standard poker variations is an A-K-Q-J-10 hand, known as Broadway straight, while the lowest-valued one is a 5-4-3-2-A straight, which is also known as the wheel.