Fortunately, the upcoming Han Solo movie will explore their time together, and Atlanta star Donald Glover will play the young Lando.
While their reunion in The Empire Strikes Back went horribly, at one time Han Solo and Lando Calrissian were great friends.
The original Expanded Universe of stories delved into their past together, but in the Disney/Lucasfilm era, those tales have been wiped from continuity, leaving a blank slate.