If these lettuce are not 'thinned' then they will spoil.
The very nature of sowing vegetable seeds in the open ground means that there are going to be surplus seedlings in the seed bed or rows. Most seed packets contain far more seed than is actually necessary for the average garden plot.
Seeds do not germinate 100%, so simply trying to sow your seeds individually in neat rows, means that there will be gaps where seeds do not germinate - or otherwise fail. There will be little uniformity as some plants develop better than others, and the onerous task of trying to place vegetable seeds individually would be very time consuming.
So, we tend to sow far more than we need, then thin the seedlings out to suitable spacing and select the strongest growing plants to leave to grow into prime vegetables.
Once the seedlings germinate, they tend to grow quite quickly if soil conditions are right, so be careful not to leave the thinning out process to late, for it will lead to week straggly plant.
How to do.
It is a good idea at the initial sowing, to sow the seed thinly and not over-sow. Some of the larger seeds can be sown as spacing instead of just scattered in the drills. A good idea is to use the 'station' method of sowing. This simply means that you sprinkle a few seeds - 3 or 4 - at intervals along the drill. Leave a gap of a couple of inches between the sowings. as a rough guide, the 'stations' can be spaced at half the eventual recommended planting distance.
As soon as the plants are large enough for you to see what is good and what is weak, the thinning of vegetable seedlings can start. This is best done with as little disturbance to the soil as possible. A hoe is often used to simply slice off the unwanted thinnings. Not the best idea. Just nip off the unwanted weaker seedlings at ground level between finger and thumb, not disturbing the soil.
Remove the Thinnings.
The unwanted thinnings should be removed from the plot or at least buried deep in the compost heap. The reason for this being that some pests are attracted by the scent of their host plant. Damaged stems and foliage such as will happen in the thinning process will emit a stronger scent than whole, health plants. Carrot fly for instance, is attracted by the scent of the carrot foliage. Don't leave it hanging about. Another little trick, is to do the thinning of vegetable seedlings during late evening when insects are less active.