Wireless internet/connection here, relating to computers. Wireless means just that, that it relies on signals instead of cables.
When data is saved to a hard drive, it's saved as binary, which is a method of "reading" using all 0's and 1's. When it's encrypted, it's basically coded using a kind of code system. To read encrypted data, you have to know the code. There are millions of different codes out there. Some people do their own encryptions (this would be very high-end level users).
Usually when you're talking about "encrypted wireless" you're talking about secure transfer of data across a wireless interface. When you're on wireless, it's easier for someone to intercept data you send out, and use it to their own advantage. Encrypting it means they can't right out read it. It's not 100% safe, but it's a lot safer than unencrypted.