Every photograph is just light recorded by your camera. The whole job of the exposure triangle is to control how much light reaches the sensor and how that light is captured. Three settings do this, and once you see how they trade off, manual photography stops feeling like magic.
Aperture: how wide the window opens Aperture is the size of the opening in your lens, measured in f-stops like f/2.8 or f/8. A low number means a wide opening that lets in lots of light and blurs the background, which is why portraits look so creamy. A high number means a small opening, more in focus, and less light.
Shutter speed: how long the window stays open This is how long the sensor is exposed, from 1/1000 of a second to several seconds. Fast shutter speeds freeze motion. Slow ones let in more light but blur anything that moves, including your own hands if you aren't steady.
ISO: how sensitive the sensor is ISO brightens the image after the fact. Low ISO (100) gives clean files; high ISO (3200+) lets you shoot in the dark but adds grain, called noise.
The trade-off that ties it together These three balance each other. Open the aperture and you can use a faster shutter. Lengthen the shutter and you can lower ISO. Change one, and you usually adjust another to keep the exposure even.
A simple starting recipe - Bright outdoors: f/8, 1/250, ISO 100 - Indoors: f/2.8, 1/125, ISO 800 - Action: prioritize a fast shutter, raise ISO to compensate
Stop trying to memorize numbers. Understand what each control trades away, and you'll set your camera by instinct soon enough.