This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Krillin's Heart of Gold side quest.
Time frame: From Android Terror Arrives - Episode 10: Intermission to Episode 10: Intermission
Reward: 206,569 EXP, 5x D Medal, Ultra Sacred Text of Martial Arts
Where to start: Southeast Islands Area, as Gohan.
Your task is to defeat the enemies guarding the cave. This won't be a problem for you - the enemies are much weaker.
Defeat the opponents to complete the mission.