Most measure TDS (in ppm) and conductivity (EC, in micro Siemens). EC would be preferable, but, unfortunately, most people use TDS...
TDS is not a standard unit but a quantity. The different scales are a representation of the electrical conductivity (EC) in specific solutions (e.g. TDS 700 is based on KCl, TDS 500 on NaCl).
You really do not need to understand what EC and the TDS scales mean. What you need to know is which scale is your probe using. If the probe measures EC and TDS, then you can determine which TDS scale is in use (it will likely be TDS 500). After that, you just need to be careful not to be using TDS values in different scales. Different countries can use different scales and, often, the scale is not stated, so you have no idea which TDS scale was used, which can generate a large error in the reading...
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TDS scales>.
No idea about the accuracy of these probes. But I have two cheap probes and a more expensive one and they all report the same values...