What does the "n" mean for pins like "nRST"?

The n in pins names like nRST means not.

• A pin named without n, for example RST or RESET, will perform its function (e.g. reset the device) when it is connected to VCC
• A pin named with n, for example nRST or nRESET, will perform its function (e.g. reset the device) when it is connected to GND

A nRESET has exactly the same meaning as overlining the name. For example $\overline{\text{RESET}}$. Some manufacturers prefer to use n instead of $\overline{\text{RESET}}$ or ~RESET