it would mean that each letter has one and only one sound, but multiple letters can share the same sound. or if it is the reverse direction for each sound you only have one letter, but multiple sounds can share the same letter. which one is true for russian i don't know.

i learned to read the cyrilic letters, but i didn't learn russian (i did try though) but with that knowledge i could read cyrilic texts aloud to someone who understands the language, assuming i learned all letters correctly and the first case is true.

in the second case i could write down anything i hear. much harder, but as a traveler that would actually be useful. be able to write down names and addresses i hear when asking someone for directions for example. i did learn to write (well, type) korean that way, but of course i had to ask a local to proofread what i wrote since i would not be able to spot mistakes.