To understand this means first of all to understand what the signal strength bars actually mean. (Content below copied from this site).
The signal strength is a combination of the actual 802.11x signal + Phone + Microwaves + other sources that might generate 2.4GHz “Noise” + Brain emissions of the user’s anxiety state (j/k), in other words it is visual representation of all the 2.4GHz in the atmosphere around the your Wireless hardware.
I.e. you might see High Signal Strength that Shows: Excellent (5 bars), but it is actually 30% signal + 70% noise.
Such a signal would be the reason for low bandwidth, and or frequent disconnection of the Wireless Network.
In contrast, a medium level Signal (3 Bars) that does not contains any noise would provide a match better connection.
Netstumbler is a free windows tool to see the signal to noise ratio.
If your router is not supported by Netstumbler, then try Kitz - DMT Tool
Db is about a relative number - a reference level if you will. So, unlike S-Unit (often used with radio operators) +db5 is literally half of 10db.
PHY is an abbreviation for the physical layer. The PHY portion consists of the RF, mixed-signal and analog portions, that are often called transceivers, and the digital baseband portion that place high demand on the digital signal processing (DSP) and communication algorithm processing, including channel codes. It is common that these PHY portions are integrated with the media access control (MAC) layer in System-on-a-chip (SOC) implementations. Other similar wireless applications are 3G/4G/LTE, WiMAX, UWB, etc. (Sourced from Wiki)