From the ohms law resistance is given by dividing the amount of current by the potential difference (voltage). In a circuit resistors may be arranged in series or parallel which determines the total effective resistance of the circuit depending on the arrangement used.
When resistance are in parallel we apply the formula 1/Rt=1/R1 +1/R2 where Rt is the total resistance, R1 resistance of the first resistor and R2 the resistance of the second resistor.
therefore, 1/Rt = 1/10 +1/10 =2/20 which when simplified is 1/10, therefore 1/Rt is 1/10, to get Rt we do the reciprocal to get 10 ohms.
Therefore the effective resistance of the combined resistors in parallel is 10 ohms