The MCP23017 is an I/O expander chip. It has 16 GPIO pins which you can control using an I2C interface using two pins from a Raspberry Pi, plus a power source and sink (which can also come from the Pi). It's not quite as simple as directly controlling the Pi's GPIO pins, but it's not complicated, either.
You need to install i2c-tools, which is probably in your distribution's package manager. You also need a kernel with I2C support; you might need to modprobe i2c-dev. It would presumably be possible to do without either of these things, and bitbang the I2C protocol over GPIO, but I don't understand the protocol well enough to try.
On pin numbering: if you like, you can refer to the datasheet for the MCP23017. There's a small dot in one corner of the chip, with a semi-circular cut-out at that end. The pin near