Yeshayahu (Isaiah) 6:1 reads:
בִּשְׁנַת־מוֹת֙ הַמֶּ֣לֶךְ עֻזִּיָּ֔הוּ וָֽאֶרְאֶ֧ה אֶת־אֲדֹנָ֛י ישֵׁ֥ב עַל־כִּסֵּ֖א רָ֣ם וְנִשָּׂ֑א וְשׁוּלָ֖יו מְלֵאִ֥ים אֶת־הַהֵיכָֽל:
which Chabad.org translates as:
In the year of the death of King Uzziah, I saw the Lord sitting on a high and exalted throne, and His lower extremity filled the Temple.
My understanding was that Hashem has no visible form. So what exactly did Yeshayahu (Isaiah) see? Was my understanding incorrect?