there are 3 modes

Volume up/down power on = bootloader

This is where you can do a factory reset(in recovery), update your OS

Volume down power on = download mode

This is where you can use certain flashing software to update the OS, or simply bring it back to stock.  Only Samsung has the official version, but you can get unofficial versions online

Volume up power on = safe mode

This is where above mentioned which runs the bare minimum and allows testing to see if apps are being bad.