What does It's time I went to bed mean?
and can we say It's time I go to bed ?
and what's the difference between the two sentences?
You can use the following constructions for "It's time":
It's time I got* my hair cut. It's rather long. — "It's time" + past subjunctive (the asterisk denotes subjunctive.)
It's time to get the tea ready. — "It's time" + to-infinitive
It's time for tea. — "It's time" + for + [noun]
From Oxford Guide to English Grammar, section 241: Tenses in sub clauses.
It's time I went to bed.
It's time I go to bed.
The first sentence is correct grammatically, but the second is not.
It's time can be followed by a past subjunctive or to-infinitive. So you can say:
It's time I went to bed or it's time for me to go to bed.
But there's a slight difference in the meaning between these forms. It's time + past subjunctive implies that I should go to bed though it's a bit late. You can add high (it's high time) to emphasize this idea.
On the other hand, it's time + to infinitive states that the correct time has arrived for me to go to bed.