Hey Cesar,
thanks for showing, what you have found here seems to be one of these nice power strips with surge protection for 8 outlets and additionally for 1 HF cable/antenna and 1 phone line, with built-in EMI filter and two USB ports as well as a resettable circuit breaker.
What is given in dB here is the signal-to-noise ratio of the built-in EMI filter.
The value of 47 dB is nothing special, but it’s good to have a EMI filter there. However this will not prevent the heavy EMI that the VFD and the spindle cable typically emit. For this, besides ferrite chokes, the most effective way is to enclose them both into faraday cages, which means an earthed control cabinet for the VFD and an earthed shielding in the spindle cable.
Image: Control cabinet suitable for a VFD.
Images: Countermeasures around a VFD for preventing EMI
As for EMI filters, some manufacturers of VFDs (like mine) offer so-called “footprint” EMI filters for their VFDs (e.g. this one for my VFD), which means you can screw the VFD onto them, they have the same “footprint” (width and height dimensions).This makes a very effective earth connection. The VFD/EMC filter assembly is then attached to the (earthed) mounting plate of the control cabinet. Will provide a photo (when my work allows it 
).
“dB” is the unit decibel, which is a tenth of a bel (B). A bel, named after Alexander Graham Bell, is a relative unit, which means it doesn’t measure a physical quantity, but expresses a ratio between two values.
The exact definition is:
A bel expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10 (approximately 1.26) or root-power ratio of 101⁄20 (approximately 1.12)."
– Source: Decibel – Wikipedia