呃
呃 can be an interjection or a sound-imitative word. Let's look at both of them.
The interjection 呃
The interjection 呃 
expresses:
1) indecision or a lack of understanding of what is going on. It can be translated as “hmm”, “um”, “well”.
expresses:
1) indecision or a lack of understanding of what is going on. It can be translated as “hmm”, “um”, “well”.
Pay attention
An interjection (感叹词) is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction. It expresses feelings and emotions but does not name them. It is usually placed at the beginning of a sentence.
Examples
呃，怎么说呢？
Hmm, how should I say it...
è, zěnme shuō ne
呃，你确定冰箱没问题吗？
Eh, are you sure the refrigerator is okay?
è, nǐ quèdìng bīngxiāng méi wèntí ma
2) admiration, surprise or a reminder. It can be translated as “wow”, “hey”.
Examples
呃，你还在这里啊！
Hey, are you still here?
è, nǐ hái zài zhèlǐ a
呃，别忘了带钥匙
Hey, don't forget to bring your keys
è, bié wàng le dài yàoshi
The sound-imitative word 呃
Pay attention
Sound-imitative words are also called "onomatopoeia" (象声词).
呃 imitates the sound of the cries of birds or a baby crying “wah”.
imitates the sound of the cries of birds or a baby crying “wah”.Examples
白天呃呃，半夜呃呃 —— 当父母真不容易
Wah-wah in the day, wah-wah in the night - it’s not easy to be a parent!
báitiān è è, bànyè è è — dāng fùmǔ zhēn bù róngyì