This is the 2nd most frequent Swedish word.
att
"Att" in Swedish means "to" (used before a verb in the infinitive form) or "that" (in certain contexts).
Here, 'att' is a subordinating conjunction introducing a subordinate clause.
Jag hoppas att det blir soligt idag.
I hope that it becomes sunny today.
In this sentence, 'att' is an infinitive marker used before a verb in its base form.
Att springa varje morgon är bra för hälsan.
To run every morning is good for health.
'Att' here functions as an infinitive marker introducing the action requested.