What does the ~ (tilde) mean for pins like “~RESET”?

The ~ in pins names like ~RST means not.

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

~RESET has exactly the same meaning as overlining the name – for example \overline{\text{RESET}}. Some manufacturers also use use n instead of ~RESET or \overline{\text{RESET}}