Feed lines, SWR and matching

Between the transceiver and the antenna sits the transmission line (most often coaxial cable). It has a characteristic impedance — typically 50 Ω in amateur radio.

Impedance matching. Energy transfer is maximal when the source, the line and the antenna present the same impedance (50 Ω). If the antenna isn't matched, part of the energy is reflected back towards the transmitter instead of being radiated.

SWR. This defect is measured as the Standing Wave Ratio, read on an SWR meter:

SWR = 1:1 → perfect match, no reflection.
SWR = 2:1 → still acceptable.
High SWR → a lot of reflected energy: losses, and a risk to the transmitter's power stage.

To correct it, insert an antenna tuner, which brings the seen impedance back to 50 Ω.

Losses & symmetry. A long cable and a high frequency increase line losses. To connect an unbalanced cable (coax) to a balanced antenna (dipole), use a balun, which keeps the coax shield from radiating (common-mode currents).

On the receive side (your case with an SDR), a good match directly improves the SNR.