An RO/DI unit is a filtration unit that essentially removes solids from the water and you get really pure water. It is largely unnecessary in freshwater tanks unless you are going for something you are trying to accomplish that would prefer softer water - like discus. In salt water tanks its necessary because of the naturally nutrient deprived reef ecosystems which cause algal blooms when there are increased nutrients, this can be caused in an aquarium by tap water.
RO/DI can be costly. A TDS meter is something you can get for relatively cheap. I have a handheld small one that looks similar to a thick sharpie and essentially the meter runs a small current between metal prongs and gives you a value depending on the amount of dissolved solids in the water. This is gathered by the varying conductance due to the dissolved solids. For example, distilled water will have a very low TDS, which might even be 0, which communicates something you might or might know, which is that pure water doesn't conduct electricity. However if you used tap water, your TDS value would be higher. If you test salt water, your TDS will be even higher once you add salt. If you are doing a high pH African tank, the TDS might be even higher.
You shouldn't need an RO or RO/DI unit unless your tap water is really poor. A TDS meter might be helpful for this, or you can use a gH kH pH testing kit, etc. A TDS meter can be fun to play with and it can be a useful tool once you get more comfortable with your tanks and can tell where your ammonia is over time and tracking the TDS can be somewhat helpful if you know what to do with it.
Sorry for the late reply!