The core idea
Your muscles adapt to the demands placed on them. If you keep lifting the exact same weight for the exact same reps forever, your body has no reason to keep changing — progressive overload is simply giving it a reason, gradually.
The main ways to apply it
Add a small amount of weight, add an extra rep or two, add an extra set, slow down the tempo, or reduce rest time between sets. You don't need to do all of these at once — picking one and applying it consistently is enough.
Why 'gradual' really matters
Jumping weight too fast wrecks form and invites injury. Small, consistent increases — even 1-2.5 kg every couple of weeks — compound into serious strength gains over months without the setbacks that come from rushing.
What it looks like in practice
Keeping a simple log of what you lifted last time for each exercise, and trying to beat it slightly this session — more weight, one more rep, or one cleaner set — is really the entire system.