一下 literally means ‘for a short time’
For example:
稍等一下
Shāo děng yīxià
Wait a moment
看一下
Kàn yīxià
Take a quick look
Another way of expressing the idea of ‘politeness’ is to repeat the verb and put 一 in the middle.
For example:
试一试
Shì yī shì
Give it a try
问一问吧
Wèn yī wèn ba
Just ask.
However, you will hear 一下 all the time in China in polite requests, and in colloquial speech.
麻烦帮我开一下门
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:
你能不能告诉我一下？
Nǐ néng bùnéng gàosù wǒ yīxià?
Can you tell me?
你介绍一下自己吧
Nǐ jièshào yīxià zìjǐ ba
Could you introduce yourself?
我想问一下，你叫什么名字?
Wǒ xiǎng wèn yīxià, nǐ jiào shénme míngzi?
I want to ask you, what is your name?
能不能帮我一下？
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!