This is the 286th most frequent Polish word.
mimo
"Despite" or "although."
"Mimo" here means "despite," indicating the situation (rain) contrary to which the action (going for a walk) was carried out.
Mimo deszczu poszliśmy na spacer.
Despite the rain, we went for a walk.
"Mimo" is used to introduce the circumstances (difficulties) that did not prevent the outcome (success).
On osiągnął sukces mimo wielu trudności.
He achieved success despite many difficulties.
"Mimo że" forms a conjunction meaning "although" or "even though," linking a subordinate clause describing an effort that contradicts the main clause (lack of success).