The title sounds familiar, isn't it ? After seeing the title it was obvious that it has do to something with rainbow tables. A rainbow table is a precomputed table for caching the output of cryptographic hash functions, usually for cracking password hashes. Basically, they are already hashed passwords used to crack password hashes. After opening the file it was indeed a some hashed strings. It can be decrypted with a online decryptor such as [CrackStation]( https://crackstation.net) or you can use any other.
Flag : ``` rgbCTF{alw4ys_us3_s4lt_wh3n_h4shing}```