The data that we encode on your NFC Tags can be locked or left unlocked :
Locked: Nobody can ever change the data on your NFC tags
Unlocked: Anyone can change the data on your NFC tags
User data is stored in NFC tags in what is called EEPROM memory. This is a type of non-volatile memory which means that the data is not forgotten when power is removed. EEPROM memory can be erased and re-encoded. In the case of the NTAG213 chips we sell, NXP (the manufacturer of the chip) state that the tags can be re-programmed 100,000 times and will store the data, without power, for up to 10 years. In reality, this isn't likely to be the case of course but the ability to erase and re-program is the important part here.
Within the NFC Chip, there's also a small section of memory which is one-time programmable. This means that unlike the main user memory, you only get one chance to encode the data and that's it. You can't change it. Within this section of memory, there's a switch (not a technical term, but you get the idea) which locks the user memory from being changed. As it's one-time programmable, once the switch has been switched, it can't be moved back again.
To lock the tags, we switch this part of memory effectively blocking the ability to make changes to the user memory. It's a simple switch and the result is that nobody - not even us - can now modify the user content. It can't be hacked. It's done.
So, until the switch is switched, the data can be changed in theory up to 100,000 times by anyone. After the switch, it can never be changed.