In a stress-timed language such as English, syllables are stressed at roughly regular intervals. Since it is the key words (typically nouns, pronouns, verbs or adjectives) that are stressed, the intervening words (typically articles, prepositions, etc.) get shortened and weakened ('swallowed'), so that two or three of them together may take up the same amount of time as the single stressed syllables preceding and following them. For example, in the statement: I like walking in the rain, the syllables I / like / wal / rain would probably be stressed and each occupy about the same amount of time as the 3 syllables kin / in / the together.
Note: There is not total agreement among linguists about the stress-timed nature of English.