What it means
It's okay, no problem. Literally "it doesn't matter" or "it has no relation."
When to use it
The standard reply to 对不起. Also used to shrug off small inconveniences.
Examples
duì对bu不qǐ起！——méi没guān关xi系。
Sorry! — It's okay.
méi没guān关xi系，xià下cì次zhù注yì意jiù就hǎo好。
It's fine, just be careful next time.
wǎn晚yì一diǎn点méi没guān关xi系。
A little later is fine.
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.