It is used to describe not knowing one’s limits and going against something beyond one’s ability. 螳臂挡车 is often used in a derogatory manner to indicate that the person will most likely fail. 不自量力 (Bù Zì Liàng Lì) is also frequently used together with 螳臂挡车 to put emphasis on a person overrating his ability. It cannot be used on yourself.
A similar idiom without the derogatory undertone is 以卵击石 (Yǐ Luǎn Jī Shí)
螳 is short for 螳螂 which means praying mantis
臂 means limb(s)
挡 means to block
车 means car
tā yǒuquán yǒu shì, nǐ xiǎng duìfù tā jiǎnzhí shì táng bì dǎng chē， bùzìliànglì
He has power and influence; going against him is suicidal and an overestimation of your abilities.