Clear space is the empty margin a mark needs so that nothing crowds it: text, edges of a page, another logo on a sponsor board (sample note).
It is measured with a part of the mark itself, called x, so it grows and shrinks with the mark. On our Kestrel Ferries mark, x is the depth of the gull’s wing.
When a printer or partner asks for your logo, send the guidelines page with the clear space too. It saves a round of proofs.