Learn Spanish (Spain): Los niños se secan al sol. - The children dry off in the sun.

Here, secar is used as a reflexive verb, secarse (to dry off or to get dry). The pronoun se shows that the children are drying themselves. If you just used secan, it would mean they are drying something else, like a towel or a dog.

Why does the sentence use al sol instead of en el sol for "in the sun"?

In Spanish, when you do something exposed to sunlight (like drying, sitting, or playing), you use the expression al sol (literally, "to the sun"). If you said en el sol, it would sound like the children are physically located on the surface of the sun itself!

Where does the word al come from in this sentence?

al is a mandatory contraction of the preposition a (to/at) and the masculine singular article el (the). Whenever the preposition a is immediately followed by el, they must merge into al.

Does los niños mean just boys, or children in general?

The masculine plural los niños can refer to a group of only boys, but it is also the standard term used for children in general (a mixed group of boys and girls).