Pictures paint a thousand words, so: a graph!
All the tall, thin areas are the 40 channels Bluetooth Low energy uses. The three orange areas are the channels devices use to advertise their presence. This is what the Loader app is listening for in order to give us a list of Beans to connect to. Once connected, all the other blue channels are used.
Note that the three most common WiFi channels—1, 6, and 11—are in the same chunk of the RF spectrum and they overlap BLE's signal space. In principal, BLE devices should avoid those channels where there's excessive interference. How this plays out in the real world is another question. But it should demonstrate that WiFi CAN interfere with BLE, but it shouldn't be a problem in most cases.
Signal strength is arguably a far more important factor in BLE performance. The RSSI (Received Signal Strength Indicator) is measured in dBm and is mainly affected by battery power and distance. Though, of course, there are also reflections, refractions, absorptions, scattering and other types of interference that can reduce (or sometimes increase) signal strength—all of which can vary over time.
You'll typically see RSSI values between -25 (antennas almost touching) to -100 (40-50 meters away) in most real world situations—less if the battery is low. Less still if there are walls, wifi, or people in the way. (People are "ugly bags of mostly water," as the silicon lifeform said, and water is a good absorber of RF.) Don't expect clean performance when your RSSI is below -90 dBm. As dB is a logarithmic scale, the difference between -30 and -31 is more than the difference between -90 and -91, but RF signal propagation follows the inverse-square law so RSSI can act like a very rough estimate of distance. (Very rough—don't rely too much on it.)
I was explaining to someone else, so I thought I'd copy and expand here.