Meaning: This sentence describes two friends who are always there for each other, no matter what.
Choice of Words: "Thick and thin" is an idiom that means through good times and bad times.
Meaning: This means that they were always there for each other, no matter what happened.
Choice of Words: The idiom "**come rain or shine**" means no matter what happens, good or bad.
Choice of Words: The phrase "**ups and downs**" means good times and bad times.