What it means
Please speak more slowly. 慢 means slow, 一点 softens it to "a bit."
When to use it
Extremely useful and always well received. Chinese speakers often speed up without realizing it when talking to learners.
Examples
qǐng请màn慢yì一diǎn点，wǒ我tīng听bù不dǒng懂。
Please speak slower, I don't understand.
qǐng请màn慢yì一diǎn点shuō说，xiè谢xie谢。
Please speak a bit slower, thank you.
nín您néng能màn慢yì一diǎn点ma吗？
Could you slow down a little?
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.