Backer rod
A foam cord pushed into a deep or wide joint before caulking, so the bead seals correctly.
Backer rod is a flexible foam cord — usually round, in several diameters — that you press into a gap before you caulk it. It does two jobs: it stops you wasting caulk filling a deep void, and it forces the caulk into the correct hourglass shape that bonds only to the two facing surfaces (not the back of the joint), so the bead can stretch and flex without tearing. The rule of thumb is to use it whenever a joint is wider than about a quarter inch or deeper than half an inch — common where a tub meets an uneven wall. Without it, a thick slug of caulk in a deep gap sags, cures slowly, and pulls apart.