Learn Turkish: Yaz aylarında erik çok ucuz olur. - Plums are very cheap in the summer months.

When making a general statement about a whole category or class of things, Turkish typically uses the singular form. Here, erik means "plums in general".

Can you break down the word aylarında? Why is there an "n" in it?

Aylarında means "in the months of". It starts with aylar (months), followed by -ı to link it to yaz (summer months). Whenever you add a location ending like -da (in/at/on) right after this linking suffix, Turkish requires the buffer letter -n- between them.

Why does the sentence end with olur instead of just saying çok ucuz?

Olur is the aorist (general present) tense of olmak (to be/become). Using olur here shows that this is a recurring fact or a general rule: plums "always become" or "happen to be" cheap every summer.

Could I use the word yazın here instead of yaz aylarında?

Yes! Yazın simply means "in summer" and is very common. Yaz aylarında means "in the summer months", which means the exact same thing but sounds just a bit more descriptive.