The ATTiny chips commonly used in Arduino-like boards, such as the Adafruit Trinket or the Digispark USB will usually have a bootloader which runs before your own code. The bootloader checks for a certain state on the USB port of the board, to see if something is waiting to write a new program to the chip.
The bootloader accepts new code from the host computer, if present, and writes it to a certain location in flash memory. If the bootloader does not find anything connected to the USB connector trying to load a new program, then it (the bootloader) starts executing from flash at the location where your program has been stored.
But, what if you want to write a new bootloader, or not have a bootloader? In that case, you can write new software using the MOSI, MISO, and SCK (clock) pins if the RESET pin is pulled to ground.
In order to make use of all 6 non-power pins as I/O for the Trinket or Digispark, a "fuse" is set in the ATTiny chip which disables the RESET pin's normal purpose of holding the chip in reset. This means you cannot pull RESET to ground and load a program the normal way.
If the RESET pin is disabled, and the bootloader is not present, then there is another method for loading software, called "High Voltage Serial Programming". Not to worry, the "high" voltage is is about 12V.
You can find schematics, or purchase kits/products that allow you to program an ATTiny chip in HV mode online. Many use the small 12V "A23" or "A27" batteries commonly found in RF remote control transmitters to get the 12V signal to put the chip in high-voltage programming mode.