Decibel is a logarithmic unit (1 dB = 1/10 Bel, 1 Bel = power ratio 1.259 = amplitude ratio 1.122) that describes a relative relationship between signals. See wikipedia for details and a table. Negative decibels mean the received signal is weaker then the sent signals (which of course happens naturally).
Level means how strong the signal is when received compared to how strong it was / it was assumed to be when sent. This is a physical measurement, and in principle the same for every Wifi hardware. However, often it's not properly calibrated etc.
Link is a computed measurement for how good the signal is (i.e. how easy it is for the hardware/software to recover data from it). That's influenced by echoes, multipath propagation, the kind of encoding used, etc.; and everyone uses their own method to compute it. Often (but not always) it is computed to some value that's on the same scale as the "level" value.
From experience, for most hardware I've seen, something around
-50 means the signal is ok-ish, something around
-80 means it's pretty weak, but just workable. If it goes much lower, the connection becomes unreliable.
These values should be read just as a rough indication, and not as something scientific you can depend on, and you shouldn't expect them to be similar or even comparable on different hardware, not even "level". The best way to learn to interpret it is to take your hardware, carry it around a bit, watch how the signal changes and what the effects on speed, error rate etc. are.