m=Milli
μ=Micro
The scaling comes from rrdtool, the graph uses prefixes from the International System of Units (SI Units)
For your Errors/Sec graph:
515.44μ is micro (10^-6), so 515.44 / 1000000 = 0.000515 Errors/Sec
4.12m is milli (10^-3), so 4.12 / 1000 = 0.00412 Errors/Sec
For example, if your polling interval is 5 minutes (300s) then your average of 4.12m is really:
0.00412 x 300 = Just over 1 Error each poll.
Looking at rrdtool it’s possible not to scale the y-axis of the graph, not sure if this is configurable in librenms yet.
B is just the total Bytes aggregate (Not related to the graph really.)