This is the 2919th most frequent French word.
extérieures
External; outer; outside.
Here, 'extérieurs' modifies 'murs', specifying they are the walls located on the outside part of a building.
La peinture des murs extérieurs nécessite un entretien régulier.
The paint on the exterior walls requires regular maintenance.
'Extérieures' is used here to qualify 'influences', meaning influences coming from outside or external sources.
Les influences extérieures ont changé le cours de l'histoire.
External influences have changed the course of history.
In this sentence, 'extérieures' is used as an adjective to describe 'activités', indicating they happen outside or outdoors.