« imo » is short for « in my opinion ».
imo stands for « in my opinion ». The variant imho adds humble — « in my humble opinion » — and is more often ironic than modest.
It belongs to written argument rather than conversation: forums, comment threads, reviews. It marks the sentence as a position rather than a fact.
Written out, it is « in my opinion », normally followed by a comma at the start of a sentence: In my opinion, the second one is better. In formal writing the phrase is often redundant — an unattributed claim in your own text is already your opinion.
Here is what CorrectSMS does with a message containing « imo ». The correction and the explanations are the app's own.
imo the first one was better, but their both fine
In my opinion, the first one was better, but they're both fine.