This type of cable tester is intended for bare cables only. Connecting it to a live network can damage the network component on the other side or (more likely) the tester itself, esp. on a PoE link.
When connected to a normal network port (again, not recommended) you measure the DC resistance/continuity across each pair that's connected by the magnetics/transformer within the port. Its DC resistance is quite low, so the tester may show continuity for the 1-2 and 3-6 pairs on a 10BASE-T or 100BASE-TX port (and likely 1-2, 3-6, 4-5, and 7-8 for 1000BASE-T). (1-2 and 5-6 indicates a miswiring fault.)
Many 10/100 ports also connect the spare pairs (4-5, 7-8) to ground, so the tester might show those as shorted.
Instead, use the tester as intended, read the manual and either connect one piece of the tester to each side of a cable (obviously disconnected from everything else), or plug both ends into the active piece. The tester checks for proper continuity, pairing, and shorts. If a problem is indicated it will impact the network connection.
However, as YLearn has already pointed out, those testers can't guarantee for the actual network function, only the lack of basic cable functions. There's much more to proper network connection than continuity and absence of shorts. The link needs to be able to properly transmit a signal of at least 100 MHz (Cat-5 for 100/1000 Mbit/s) with very little crosstalk. A proper network tester checks all parameters of a cable (see Ron's link in his comment) and starts at $600 or so, depending on the cable grades it supports.