This is the 753rd most frequent Swedish word.
varnar
"Varnar" means "warns" in English.
In this sentence, 'varnar' is used to indicate that the police are giving a caution to the drivers about the state of the roads. It is used in the present tense, suggesting an ongoing action.
Polisen varnar bilister för halt väglag.
The police warn drivers of slippery road conditions.
Here, 'varnade' is used in the past tense form to describe an act of caution given at a specific time in the past, directed towards the speaker about a particular action.
Hon varnade mig för att gå ut i stormen.
She warned me against going out into the storm.
This use of 'varnar' again reflects an active warning being communicated in the present, with researchers being the subject cautioning others about global environmental issues.