简化 (jiǎnhuà) means 'to simplify' in a general sense — making something less complex or easier to understand or do. 精简 (jīngjiǎn), its near-synonym, adds the idea of 'streamline/downsize' — cutting out unnecessary parts, often for efficiency (e.g., reducing staff or bureaucracy). Use 简化 when the goal is reducing complexity; use 精简 when the focus is on cutting to the essentials.