Cherries, On the weights you can either do low rep/heavy weights or high reps/low weights.
8 is about the mid ground in terms of reps. You should do three sets, which is lift the weight or do the exercise 8 times, take a 90 second break. Do it another 8 times, take a 90 second break , do it the final three times. So things like deadlifts , or weighted squats or lunges, weighted v sit ups ( you raise back and legs to make your body into a v whilst pulling the weight over your head) Russian twists, whatever,
The difference is heavy lifting with low reps, builds strength, light lifting with high reps, builds endurance. Of course At some point strength becomes endurance. So arguable which is best.
So three sets of eight reps with the heaviest you can manage is the mid ground between the two and perceived as optimal between strength and endurance.
So when I say lift as heavy as you can, I don't mean lift something you can lift once or twice and that's it. That's low rep heavy weight. And you're not lifting something you could lift 20 x 3 sets.
You're lifting something that is heavy enough that on the last lift, on that last set of eight , you struggle to do it. The first lift on the first set of eight, it will feel quite light, but once you've done it 24 times, (3x8) it starts to feel very heavy indeed and your muscles will struggle to do it.
So I should probably have said, lift as heavy as you can to be able to do three sets of eight reps, and the last one or two lifts should be a struggle and you should feel like you couldn't even do one more.
Not sure if that makes sense 🤔
On a side note I made broccoli and Stilton soup, it's so easy! Very low carb and was actually delicious. 😇