Understanding 75 ohm antenna connection on a hifi receiver
Yes, you should assume that the receiver is a 75 ohm load, with one conductor grounded (i.e. unbalanced).

Higher frequency signals will create reflections if they try to go through a point where the impedance does not match. A system where the source, load, and cables all have matching impedances will not produce any reflections (and it will also have maximum power transfer).

For sources, the source impedance is similar to a resistor in series with the source. For loads, it's similar to the resistor connecting the two conductors.

For cables, the concept is more complicated. Wikipedia has a good overview, but the rough idea is that the cable's impedance is the ratio of voltage to current in the cable. Common cable impedances are 50, 75, 150, or 300 ohm.

The antenna design software you use is designed for both transmitting and receiving signals, but transmitting is often more important, so they call it the source impedance. But, the antenna being a symmetrical device (from the transmitting/receiving point of view), it is equivalently the load impedance.

Generally, designers will provide purely real source and load impedances, so you don't need to worry about the imaginary parts of the source/load impedance. Note that the impedance of the antenna is a function of frequency, and your antenna will not be able to have a purely real impedance at all frequencies. Usually a Smith char (polar plot of impedance) is used to visualize impedances. If your impedance is wrong, then a transformer (either coils of wire or quarter-wave lines) could be use to match it to the 75 ohm load.