Seeds need the right amount of moisture and heat to properly germinate, and typically they need more moisture and heat to germinate than they do to grow.
Poor germination is a common problem with starting seeds whether in seed flats indoors or direct seeding into the ground. When you presoak your seeds, you can reduce the time it takes for them to germinate making it less likely they will rot in wet soil if temperatures are less than ideal. It also uses less water overall as a well pre-soaked seed can still germinate and grow well in drier soil.
Which seeds should you pre-soak?
Any seed will benefit from pre-soaking, but generally bigger seeds are worth the extra effort of pre-soaking to me. Seeds like peas, beans, squash, watermelon, and corn.
How long do you pre-soak them?
I usually pre-soak my seeds 1-4 hours. It depends on the size of the seeds (bigger seeds = longer soaking), how dry they are, and my patience. Some folks do it much longer, but too long and the seeds will send out their tap root which is easily damaged. I don't like having to be super careful while planting so I don't wait that long. Experiment to find what works best for you. Even a short soak while you are preparing your seedbed can give your seeds a worthwhile boost.
How do you know they are ready?
As the seeds imbibe the water, they will fill back out and look plump. They will also become softer, so be gentle with them. If they send out a root, I feel that is too much time soaking but you can still plant them then as long as you are careful not to damage the taproot.
What else should I know?
Start with cool/room temperature water, then after 15-30 minutes add warmer water. Stir or swish occasionally to keep the seeds oxygenated. Warmer water will speed up the process, but always start with cool/room temp and gradually increase the temperature-- and never use hot water.
If you do more than one type of seed at a time, put them in separate containers and label the containers. You can be preparing the planting medium/soil while the seeds soak.
If you try pre-soaking your seeds this year, let me know how it goes!