In short, it’s like bike gears, the higher the gear the faster you go but the harder it is to push. So if 2 of the same reels states a 5:1 ratio this means 1 full revolution of the handle makes the bail arm wind around 5 full revolutions in the same time, giving good speed but looses out on lifting power. As a 1.5:1 means 1 revolution of the handle makes the bail arm wind around slower but giving a better gear ratio to lift big weights making the job a bit easier.
So when speed is needed for say poping lures, a large spool size coupled with a high ratio is the way to go. When lifting power is needed then a lower ratio is the way to go. But what if you need both? Well you need a 2 speed reel but these are only found in an over head style but great bits of kit.
The Sydney Marine Man