While there can always be some ambiguity, Hebrew and other Semitic languages have a system of triconsonantal roots, in which each sequence of three consonants suggests the meaning of the word. For example, the root k-t-b, meaning "to write", is used to derive words like kāṯaḇti כתבתי "I wrote", kāṯaḇ כתב "he wrote", kattāḇ כתב "reporter" (m), kəṯāḇ כתב "handwriting", kəṯōḇeṯ כתובת "address", and kəṯīḇ כתיב "spelling" (m).
As you can see, several of these have identical or very similar spelling, so there is some amount of guessing based on the context (the sentence "a reporter wrote about his handwriting": "כתב כתב על כתב ידו", has the word "כתב" repeated three times, but one can probably guess it's not saying "a handwriting reporter-ed about his wrote"), but as unlike in English, triconsonantal roots are an inherent part of Hebrew, some of the context is helped by recognizing the k-t-b root.
To use your example, the reason "rd" wouldn't be as understandable in English is that the history of English vocabulary includes vowels and has many words with the same consonants, if English had had the same morphological system as Hebrew it's likely we'd have the root r-d connected to, say, the general meaning of "road", and words with an r-d root might include things like "road", "pavement", "asphalt", "carriage tracks", etc., while words like "read" or "rude" could be folded in under other roots with generalized meanings like "to read" or "to insult".