Removing all but the strongest seedlings to allow plants lots of room to grow, and good access to moisture, nutrients and light. These are the key requirements for healthy plant growth.
Not all plants handle thinning the same way. Those with fragile roots, like beans and cucurbits (watermelons, pumpkin, courgette, cucumbers), should be thinned as soon as possible, before their roots have a chance to become intertwined with one another, or else the remaining seedlings may suffer from root disturbance. Gently pull out the unwanted seedlings, leaving the healthiest in place.
Root crops are more sensitive to thinning and should be pulled out with extra care, or even cut at the soil line. Depending on the plants and their mature size, spacing may vary. While most people prefer a finger width between and around seedlings, you can’t go wrong with using two finger widths. It’s always better to be safe.