This is the 12330th most frequent Swedish word.
simma
To swim.
Here, "simma" is used as an infinitive verb to express the activity of swimming in general.
Jag gillar att simma i sjöar.
I like to swim in lakes.
In this sentence, "simmade" is the past tense form of "simma", illustrating a completed action of swimming.
Han simmade snabbt över bassängen.
He swam quickly across the pool.
The word "simmat" represents the past participle form of "simma", used here to describe an ongoing action completed over a period.