You need to Lift heavy things.
Heavy weights make you stronger. Simply put, if you can only lift something a few times (say, <5), the act of lifting it will force your body to compensate by making you stronger. This is a gross simplification, but generally true. Lifting things you can lift many times don't make you as strong as lifting things you can lift only a few times. This does include your body - pushups, bodyweight squats, handstands, etc. are absolutely "lifting heavy things" if you can only do a few of them. Or zero of them.
Heavy weights require technique. If you lift something light, you can usually lift it incorrectly without a problem. Bend over and pick up a golf tee off the ground, and the weight is inconsequential. You can lift it any old way. Lift a 10 pound bag of salt, well, now you need to pay more attention to your stance and grip. Make it a shifting 10-gallon (85 pound!) jug of water and you better engage your body correctly.
So lifting heavy weights forces you to check your stance, your grip, your footing, your back position, to breathe properly, and more. If you don't lift it correctly, it won't budge. Thus you learn to lift correctly because nothing less will do.
Heavy weights expose weaknesses. If your grip is weak, you can't lift something too heavy to grip. If your legs are weak, they'll fail you on a heavy lift. Arms? Same thing. Lifting heavy things will show you what you need to fix in a way light weights can't. If it's too light, your strengths will mask your weaknesses by compensating. Get heavy enough and they'll strain all the muscles that should be involved, and the weak ones won't do their job. Only then do you really know where your weakness lies.
We all lift heavy things in life. Or we end up needing help to lift them. It's better to be able to lift them yourself. You'll never know when you will have to, so be prepared.
What do I mean by heavy, anyway?
They only need to be heavy for you. Heavy isn't absolute, it's relative. For Magnus Samuelsson a 225 pound deadlift is light. For a new trainee, that's quite heavy. For an elderly untrained person, one pushup might be relatively heavy. For Jack Lalanne, it was just the start of the warmup. So lift something that is heavy for you.
So go lift something heavy next workout!