This is the 480th most frequent Russian word.
летом
"In summer."
"Летом" is used here to specify the time of the activity (during summer).
Летом мы ездили на фронту природы.
In summer, we traveled to a nature reserve.
"Летом" indicates the season in which the subject anticipates the activity (in summer).
Я всегда жду летом возможности искупаться в реке.
I always look forward to the opportunity to swim in the river during summer.
"Летом" is employed to denote the season during which the events take place.