一下 literally means ‘for a short time’
Shāo děng yīxià
Wait a moment
Take a quick look
Another way of expressing the idea of ‘politeness’ is to repeat the verb and put 一 in the middle.
Shì yī shì
Give it a try
Wèn yī wèn ba
However, you will hear 一下 all the time in China in polite requests, and in colloquial speech.
Máfan bāng wǒ kāi yīxià mén
Could you help me to open the door?
In this situation, you can’t translate 一下 as ‘for a moment’, it’s just used to make the sentence ‘softer’ or more polite
Some more examples:
Nǐ néng bùnéng gàosù wǒ yīxià?
Can you tell me?
Nǐ jièshào yīxià zìjǐ ba
Could you introduce yourself?
Wǒ xiǎng wèn yīxià, nǐ jiào shénme míngzi?
I want to ask you, what is your name?
Néng bùnéng bāng wǒ yīxià?
Could you help me?
一下 is used a lot by native speakers, so try to listen out for when you hear it in conversations, and after a while you will start using it more naturally yourself!