What does it really mean?
Received signal strength (RSSI) on client side (how strong is the router’s signal at the client)?
Received signal strength on the router side (how strong is the client’s signal at the router)?
May be it should show both?
Or it is really relation Tx power to the Rx power? Why it is negative in this case? Tx power should be much more than Rx power…
Can anyone explain, please?
I’ll assume that units in winbox are right …
Whenever some quantity is expressed in dB, this means logarithm (base 10) of ratio between two values with same unit … which makes ratio unit-less. E.g. “my index finger is 3dB longer than my child’s index finger” … in decimal units that translates into “my index finger is approximately twice as long as my child’s index finger”. If original ratio value is less than 1, then the “dB” value is negative.
Then we come to electrical engineering, radio in particular. Signal strengths refer to power (which flows through antenna, either Tx or Rx), which in principle is measured in Watt. More often we’re using miliwatts and power, expressed in logarithmic units, has unit of dBm (deciBell milliwatt) because it represents decimal logarithm of actual power divided by 1mW.
If (Rx) power is less than 1mW (and in telecommunications it almost always is), then the value (in dBm) is negative.
Now to the value shown in winbox: my experience is, that it represents Rx power, received from link partner. AP shows Rx power of all asociated clients and client shows Rx power of its serving AP. In WiFi Rx signal strength doesn’t get reported back to transmitter. As technology used is TDD which means it’s safe to assume that pathloss is the same in both directions. It is also safe to assume that both link partners are transmitting at comparable power (a few dB higher or lower doesn’t make much of a difference). In some other technologies (e.g. LTE or 3G) feedback about received power is essential because power control (to reduce interference) needs the information. But even in those technologies information about Rx power sent back to transmitter may not be absolute value.
Thank you for good explanation, but I know what dB and dBm are… My question was not about units, but about value itself.
If I have understood you correctly, your point is this value is Rx level on router’s side, signal strength as router see it from client. That is what first come to my head. too 
. But why they call it “Tx/Rx Signal Strength”? Based on idea “it’s safe to assume that pathloss is the same in both directions”?
If wireless interface is TDD (i.e. nstreme instead of 802.11), then “Tx/Rx Signal Strength” actually shows two values. Or it could be because one unit is configured as bridge and the other as station-bridge.
As they are both deeply negative (in my case -58/-60) I assume both refer to Rx level … the other being value reported back by PtP peer.
The “it’s safe to assume that pathloss is the same in both directions” referred to text following this sentence … essentially about lack of power control in WiFi