Learn Spanish (Latin America): Cuando mi suegra vaya a la playa, descanso un ratito. - When my mother-in-law goes to the beach, I will rest for a little while.

This is the main rule of your current lesson! Because the mother-in-law going to the beach is a future event that hasn't happened yet, we must use the subjunctive mood after cuando. So, we use vaya instead of the normal present tense va.

Which verb does vaya come from?

It is the present subjunctive form of ir (to go) for ella (she, referring to mi suegra). The verb ir is very irregular, which is why its subjunctive form looks completely different from the infinitive.

The translation says "I will rest," but isn't descanso the present tense?

Yes, descanso literally means "I rest." In Spanish, the present tense is frequently used to talk about the near or certain future. You could also say voy a descansar (I am going to rest), but using the present tense here is very common and sounds natural.

What is a ratito?

It comes from the word rato, which means "a while" or "a bit of time." By adding the common ending -ito, we make it smaller (a diminutive). So, un ratito just means "a little while."