Pain or discomfort is not something to be afraid of. It is the body's way of telling us something different is happening. And if we always hide from that, then this can cause the body to become sensitised to the pain/discomfort.
But it doesn't mean we push and push and push. It means we are sensible about how much discomfort is ok.
There is evidence to suggest that exercises need to be painful in order to work. But there is also research suggesting that exercises should NOT be painful in order to work. And chances are the research will continue to change all the time.
Using the traffic light scale can help us to know when we can do more and when to rest a little (or at least slow down a bit!).
So, for example, if you are doing something and feeling like a 1-4 during and 24 hours later, then great. You can probably go right ahead and do more.
If you are hitting a 5-6 during or up to 24 hours afterwards, then I suggest maintaining that amount. That is, until your body gets used to it. Then you can go ahead and increase.
And if you're hitting 7+, you may have completed too much too soon. Take some active rest or slow down a little. Do this until the level drops a bit.
So, this isn't a perfect science; yes, it is subjective. But the whole point is to let you know that SOME discomfort is not a problem. But knowing when you can push a little more and when to back away is key.