what are the plaintext's for the RC4 stream cipher test vectors in RFC6229?
RC4 is some kind of pseudo random number generator. You input a key (seed) and now can get a stream of pseudo random numbers. The values here are this numbers, always a byte at a time.

Unlike a block cipher, you don't have a plaintext. You use this key stream to add it to the plaintext to get the ciphertext. "Adding" can be done with XOR, because that's pretty easy and the most common way.

Yes, DEC means the offset. The first 16 values are the 16 output bytes of RC4 with the mentioned key. The next 16 values are the output bytes after the first 16. The third line has the output bytes after the first 240 bytes.

We could say that the plaintext was all zero bits. This would lead to the ciphertext being the key stream.

This online tools allows to encrypt / decrypt something with RC4. Switch the input and the key to hex numbers, write only zeros as input text and then copy the key into the key field (without the 0x at the start). Now you can see exactly the mentioned numbers as output of the function.