To understand what a resistor is, we need to know what voltage and current are. Water analogy is a popular analogy to understand voltage, current and resistance. Think of the battery as a water tank full of water. Now imagine a pipe attached at the bottom of the tank. If the pipe is open the water will flow out due to the "potential difference". The water always flows from a region of higher potential to a region of lower potential. The same is with current. In the water analogy the potential difference determines how much water will flow out of the pipe. The pipe also plays a major role in determining the volume of water that flows out. The water that flows out can be compared to electric current and the pipe can be compared to a resistor. Thicker the pipe, less would be the flow of current. Therefore, resistors are passive components that resist the flow of current and decrease its magnitude.
This post was just meant to be theory. We will be doing more math and applications of resistors in the upcoming posts.