Look at these examples:
In English, we often simply squish nouns together to form new words for things: wine glass, snowball, ironing board, etc. (we call these compound nouns because they are formed from two nouns compounded together).
In French though, compound words like this are almost always separated by à or de or en and the meaning changes depending on which is used.
à usually indicates what something is designed for, e.g. un verre à vin.
Note that to say a glass of wine, we use de where English uses of: e.g. un verre de vin.
de / en
de or en usually implies what something is made of, e.g. un sac de cuir / un sac en cuir.
à la / à l' / au / aux
Additionally, à la / à l’ / au / aux are used particularly with food to mean with, made with or flavoured with, e.g. une glace à la vanille.
Also have a look at Compound nouns formed with noun + à + verb-infinitive