What are round numbers? Like $4,500, $4,000, etc.?
That's right.
There's nothing scientific or guaranteed about it, just something I've observed. And the more 'round' the number (ie divisible by a larger round number, such as $1000) the greater the resistance seems to be (eg: $4500 will tend to see more resistance than $4400. And $5000 will see more resistance than $4500). It's human nature I guess; they're psychological milestones. And people who are looking to profit from a price increase are more likely to set an automatic sell order somewhere near one of those milestones than choose some arbitrary number. Of course the milestone isn't always in USD, sometimes you need to pay attention to other influential fiat currencies like EUR or CNY.
The inverse seems to be true too when looking at the falling price and the 'support level'. If you look at the Bitcoin price chart (below) for the last 7 days, notice how the last couple of times the price fell it hit a new support level of around $4000. That's partly due to the automatic buy orders and people thinking "if it drops to $4K again, I'll buy some more".
Can you recommend a good resource for learning about cryptocurrency markets?
I don't really have any to be honest. I'm just an amateur at this myself. Most of the time I just do a little research and watch the price, looking out for a pattern. There are lots of resources that'll teach you how to work the data like a stock market pro, predicting the price by studying candlestick charts but I'm not convinced that works. Cryptocurrencies don't behave like stocks and shares; they're highly volatile and are influenced more by awareness, hype, development activity and the size of the community or following.
When I'm looking for a cryptocurrency to invest in, most of the time I just research them by Googling and checking out their info on Coinmarketcap, where you can find links to their announcement (ANN) thread and website, information on price, market cap, supply, daily volume and the exchanges they trade on. If I'm looking for a longer term investment, I look for something that has real world 'value', something that provides an innovative solution, product or service. Depending on how much I'm planning to invest, I may study the whitepaper, the project's history and check out the project developers. From that information and the 'scarcity' of the coins (ie the maximum supply) you get a feel for where the price should/could be. I'll usually invest if the coin is relatively new and seems promising but is currently very undervalued. Having said all that, sometimes I just take a punt on a new, low priced coin that has a lot of hype and is likely to get 'pumped', though those are a more short-term investment.
There's a guy on YouTube who goes by the name
Suppoman who makes daily investment advice videos. To be frank, he's a bit of a loud shouty plonker but he's quite entertaining to watch and he does do his research. His advice on which cryptocurrencies to watch and invest in is usually quite good. He does have a 'course' you can pay for too, which costs about $10 (if you use the link he provides with his videos). I would recommend doing your own research before investing in any that he mentions though. And, as always with cryptocurrencies, the best advice is
never invest any more than you can afford to lose.