Aller chercher literally means “to go to look for”, however, it is also used for going to fetch something, or to pick someone up from somewhere. As is normal with these types of construction – those that use the semi-auxiliary verb “aller” – only aller is conjugated, and chercher remains in the infinitive.
Je vais chercher mes enfants à l’ecole – I am going to pick my children up at the school
Il va chercher du pain – He is going to get some bread.
Ils sont allés chercher du lait – They went to get some milk.
Elle est allée chercher son mari à la gare – She picked her husband up at the station.
Ils vont chercher ses clés – They have gone to look for his keys / They have gone to get his keys.
Note the past participle agreement here when used in the passé composé. Because aller is conjugated with être as it’s auxiliary verb we have to make the past participle of aller agree with the subject. So if the subject is feminine then we need allée (Elle est allée chercher…) if the subject is masculine, it’s allé (Il est allé chercher…) and for plural masculine and feminine subjects it’s allés (Ils sont allés chercher) and allées (Elles sont allées chercher) respectively.
These are all pronounced the same though, so this agreement only needs to be made in writing.