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I feel dumb for asking this question.. but does rainwater contain mineral? because I collect it from the roof/edge drain
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I have read this "The TDS meter calculates a general conductivity measurement that depends on the overall amount of minerals dissolved in the water.", and so I bought one...

I measured the following
- Distilled water (1 ppm)
- Rainwater (2-6 ppm)
- Tap water (53-60 ppm)
- Kirkland(costco brand) Purified water [with mineral added for taste] (20-23 ppm)

I guess what I'm trying to ask is …

1 - Do I just trust the TDS meter?
2 - Is the Kirkland purified water safe? (don't get me wrong, I'm definitely not using it)
3 - If you have a TDS meter, have you tested distill water and is it the same result as mine?

Well, to start with, don't feel dumb for asking questions. We've all been there, that's how we learn. In fact, I learned something new today from this forum on another topic.
I test my TDS tester by measuring distilled water, and it always reads 0. So yours reading 1 is good.
The rain water might pick up dust from the air on its way down, but it definitely picks up stuff from the roof. However, at a reading of 2 to 6, use it.
The Costco water with added minerals I wouldn't use. The minerals are two types of sodium, potassium, magnesium, and calcium. If it the only water available, use it. Any water is better than no water
Lastly, at 53 to 60, you could actually get away with using tap water. I know a guy that uses tap at around 100 ppm, and he doesn't usually have an issue. But I would gently top water at least once a month to flush any buildup of minerals due to evaporation from your tray.

Walmart brand distilled - 6 (i was shocked)
rain water off the metal roof of my barn - 2
tap, yeah i am lucky - 24
zero water filter - 0

On my inside plants i still use rain water or filtered water only, I am still on my 1st zero filter i got last fall. I think with low TDS tap It last a long time.

A cheap TDS meter isn't going to be perfect but close enough for what you need. Distilled water should always be 0 ppm. If you put it in a glass to test it, it is possible that the glas contaminated it. Added minerals means salts and salts are bad for CP. Rainwater may pick up some contaminants from the roof but that is why you test it. At 60ish ppm for your tap water, you could probably use that if the plants are regularly flushed with rainwater. I used my tap, which was 72 ppm exclusively to water my temperate CPs but they were grown outside and were regularly flushed with the rain. Also, the minerals were not high in calcium (the white scaly stuff you get in your sink) which was also good.

As an aside, Reverse Osmosis filter followed by a deionizing filter produces 0 ppm water also.
You can find some on Amazon for around 60 or so USD.

Rainwater collected from the roof usually has a slight mineral content, especially if the roof has asphalt shingles (like mine). I wait at least 15-20 minutes for the roof to get washed off, then start collecting. My rainwater hits 5-10 on my TDS meter.

thank you all for providing the knowledge

My roof rain water is typically around 1-3 ppm. Rainwater does contain some nitrogen. Which is good for leaf growth. It's why gardens often look like they grew a lot after a good rain.

Living in a city, my rainwater is about 12-20, it all depends on where you live, the rainwater will pick up air pollution on the way down plus the dust in the collection device or on the roof, but it is always OK for the plants.
Cheers
Steve