This is the 90th most frequent Slovenian word.
letu
"Letu" in Slovenian means "year" (locative or dative singular form).
In this sentence, 'letu' is used in the locative case of the noun 'leto' to indicate the time within the year 2020.
Letu 2020 je bilo veliko pomembnih dogodkov.
In the year 2020, there were many significant events.
Here, 'letu' again appears in the locative case to specify a particular year during which actions occurred.
V tem letu sem se veliko naučil.
In this year, I learned a lot.
This usage highlights the relationship to 'leto' in discussing time periods.