Answer: The purpose of thinning is to keep the plants from being overcrowded and stunting each other as they compete for light, water and nutrients. If, for example, you planted three seeds in one cell of a sixpack and they all sprouted, then you will need to remove two of the seedlings. usually, one of them simply looks better -- bigger, stronger, more robust or vigorous and that's the one to keep.
You have several choices about how to do it. The fastest and least disruptive method is to snip off two of them at the base using a tiny scissors (mail scissors are great) -- remove the trimmings and throw them away. You're done.
Another method would be to pull two of them out by the roots doing the best you can not to damage the seedling you want to keep. This is a bit rough on the pulled plants but you might be able to transplant them nonetheless.
Finally, and this is usually only done if the seeds are very expensive or there is some other over-riding reason to want to preserve all the seedlings, you can salvage and transplant all three of them. To do this, remove the entire soil mass from the pot and gently drop it onto the table so that the soil and roots fall away from each other. Gently remove each seedling (hold it by a leaf, not the stem) and then replant (one to a pot) in clean new soil and water to settle them in. This is very painstaking work but it can usually be done with a high success rate.
I hope this helps!
Q&A Library Searching Tips