This is further explained in Solo: A Star Wars Story: Expanded Edition, and in short it is a restraining bolt to keep the craft where it is, like a wheel clamp in our world.
Han clears his throat. "Looks like you've had some work done," he said, stroking the metal shielding.
"Indeed I have, Han," Lando said, still mispronouncing his name. "Two years restoring, retrofitting, adding features: customized flux regulator, new alluvial dampers, a wet bar..."
"...a fortified infraction restrain on the landing gear?" Beckett added to the list from underneath, looking at the ion restraining bolt affixed to the front strut.
Lando feigned surprise very poorly. "What is that doing there?"
Beckett sighed. "Looks like someone fell behind on their docking fees," he said.
Solo: A Star Wars Story: Expanded Edition