Think of a frame whose legs spread at the bottom, with a bar across the top, standing there empty. That shape is the whole word. The legs spread, so weight presses down and and nothing tips; the middle is open air, so the frame is light, cheap in material, and can be carried, folded or built out of a few beams. Whatever you lay across the top — a board, a table top, a railway track — the trestle is the frame underneath, never the thing it holds.
From the word’s history
Middle English trestel, from Old French trestel meaning crossbeam, from Latin trānstellum, a small trānstrum (crossbeam). The oldest idea in the word may be the beam lying across the top, with the spreading legs coming to be counted as part of it later.
中古英文 trestel 來自古法文 trestel（橫梁），再往上是拉丁文 trānstellum，也就是 trānstrum（橫梁）的小稱。所以這個字最早指的可能就是橫在上面的那根木頭，後來才把下面張開的腳一起算進來。
Wiktionary, "trestle"