In English we often simply squash two words together to make a compound noun ("dining room"), whereas in French the two words are always joined by a preposition (à, de, en).
Look at these examples:
une planche à repasseran ironing board
une machine à lavera washing machine
une machine à coudrea sewing machine
une pomme à cuire a cooking apple
Compound nouns in English formed with -ing + noun are often used to indicate function (e.g. "dining room"). In French, these typically take the form noun + à + verb-infinitive. For example, salle à manger.
Also have a look at Compound nouns formed with prepositions à, de, en
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