Resistor in series We know that when a resistor is connected in series , the current flowing through that resistor will be constant.But how do we use a resistor to limit the current flowing through a circuit ,when resistor is connected in series with the circuit ?
 A: Resistance can be interpreted in various ways depending on the circuit. It can be used to cause a potential drop, or it can be used as a heating device, etc.
You are asking how resistance can change the current flowing through the circuit when connected in series. In that context, the resistance can be used to alter the total resistance of the circuit which affects the current flowing through the circuit.
Ohm's law states that for small voltages (up to 1000V - depends on the device actually), ohmic devices obey the following relation
$$I = \frac{V}{R}$$
For a simple circuit such as the one given below, the total resistance decides the amount of current flowing through the circuit.

Here the E.M.F (voltage of the source in an ideal battery) is fixed and what the turns up in the denominator decides how much current will flow through the circuit.
Real-life Application (Plugging an LED to your 12V battery)
Let's talk about a more practical application, say you have an LED bulb of which has a resistance of $2 \Omega$. If you plug it to your 12V battery, the LED is going to blow up because it would draw nearly 6A which is WAY too much.
So does this mean you can't use LED with your battery? No. You can use by connecting a resistor in series with the LED.
The LED needs just a little bit of current, let's say it is rated to operate at 1A of current. 

You want the total current flowing through your circuit to be 1A. The total resistance you'll need in the circuit will be given by,
$$R = \frac{V}{I} = \frac{12V}{1A} = 12\Omega$$
$$R = 2\Omega(LED) + R_{resistor}$$
$$R_{resistor} = 10\Omega$$
So you will have to connect a resistor with the resistance of $10\Omega$ in series with the LED to ensure that nearly $1A$ of current passes through the LED.
