That is an Airport Wireless Express ( portable) base station that you plug into a wall outlet, plug your Ethernet cable from the modem you're using to get your ISP signal.
This sends out the WiFi signal from this small box.
It will, probably, work, but you still need some sort of USB adapter or Airport card to receive the signal.
If this is an early model eMac, you'll be relegated to Wireless "B" speeds only.
Since you either purchased this or were given this, I am curious. If purchased, Why an eMac? And why bother with an early model one at that?
This eMac is the slowest of the line.
What kind of budget were you on?
There are lots of used/newer Macs out there with a lot better specs.
If you're just getting into Macs, this will not be a very good first impression/experience for you.
The OS is not the best OS for this model eMac combined with the dog slow, nearly dial-up speeds of the wireless "B" specs.
We can give you more guidance about how to maximize some of the performance of this eMac, but, in this day and age, it's still going to be a turtle.