I'm not sure if this will show properly for you, but worth a shot (I don't normally use Yahoo Finance, the tool I normally use is paid so I can't link):
https://finance.yahoo.com/chart/%5EGSPC ... UiOm51bGx9
If that shares the chart exactly as I marked it, you should be able to see some lines I added which are major support/resistance levels in the S&P 500 going back to the start of 2018. You might have to zoom out a bit to see all of 2018 to now. They are pretty clear and easy to see when you know what you're looking for. The market tends to turn around often in the same areas. Now, it's important to note that even though I gave 2820 as a hard resistance level, that these things are generally more of a range than an exact number. That 2820 one has seemed to reject pretty tightly in the past, but sometimes a resistance level can be something like a 50pt range.
You should see that on the chart, that the market found support and resistance in certain general areas repeatedly. Also, it's worth mentioning that once one clears a resistance level, in the future it tends to act as support. So these areas are simply major inflection points in the market. The big decisions on where the market is going to go next tend to happen in and around these levels. The bulls and bears go to war around these levels, and the side with weaker conviction loses. The winning side takes temporary control of the market, while the other side licks their wounds for a bit. This is not a coincidence. It happens all the time in both individual stocks and general markets.
Now you'll also see that support and resistance also don't always put up a fight, sometimes, like in December, the momentum is so strong that it just rips right through it. So like I mentioned earlier it is not predictive. One can assume that a prior support/resistance level will provide the same support/resistance in the future, but it's not always the case. It generally takes a lot of momentum to break support or resistance, which is why I am skeptical of todays breakout, because momentum (measured by MAC-D which is not on that chart) has been trending downward for 2 months now. The bulls don't seem to have a ton of conviction right now, like they had in early January. Also, small-caps, which generally tend to lead the market, have been weak. Bond markets, which have a better track record at predicting major economic turning points than the stock market, have been looking bearish. All things considered I think this is going to be a false breakout. But that's just my opinion, I could be wrong. These things provide clues, but there are always conflicting clues, and nothing that can definitively predict what is going to happen. Also, momentum measurements like MAC-D are always looking to the past, and new catalysts could enter the market to increase momentum (like a trade war or Brexit deal). We will have to wait and see.
If I were actively trading, I would be entering a trade today good until cancelled to go short when/if we break below 2810, and if that executed I would put in a stop buy at probably 2840 or so to keep my potential losses low. I would be hoping to get down around 2630, the next support level, to cover (or at least move my stop buy down and see if I can ride it even lower). That, if successful and stopped out a bit above 2630, would be a little bit over a 6% gain, more if I used leverage or if the market broke support and went lower. Limiting the downside risk if wrong first and foremost, but allowing the option for gains to run a bit if right. Defense first, offense second.