About this passageA string of separate instructions from Hillel, kept together in the tractate as he is said to have given them. The one about judgement uses a spatial image — literally, until you have arrived at his place — and the tractate offers no test for when that has happened. The last two undercut a person's confidence in their own account of themselves: that what is obvious to you will become obvious to your hearer later, and that the time to do this properly will present itself.
Hillel said, "Separate not thyself from the congregation; trust not in thyself until the day of thy death; judge not thy neighbor until thou art come into his place; and say not anything which cannot be understood at once, in the hope that it will be understood in the end; neither say, 'When I have leisure I will study'; perchance thou wilt have no leisure."
In plainer words
Stay with other people. Do not assume you have yourself figured out, not for as long as you live. Do not pass judgement on somebody until you have stood where they are standing. Do not say something obscure and expect it to come clear later. And do not say "I will get to it when I have time" — you may not get the time.
A restatement in modern English, written for this collection. The passage above is the text; this is one way of putting it.