This is the 1549th most frequent German word.
falls
"Falls" means "if" or "in case" in English.
In this sentence, 'falls' is used to introduce a conditional clause, indicating a possibility or hypothetical situation.
Falls es morgen regnet, bleiben wir zuhause.
If it rains tomorrow, we will stay home.
Here, 'falls' is again used to express a condition under which the action of letting him know will occur.
Falls ich ihn sehe, sage ich ihm Bescheid.
If I see him, I'll let him know.
In this context, 'falls' provides a conditional premise that defines the obligation described in the main clause.