Skeletron is a major pre-Hardmode checkpoint because defeating it opens practical Dungeon progression. The encounter is summoned through the Old Man at the Dungeon entrance at night.
Why Skeletron matters
Winning the fight turns the Dungeon into a major source of enemies, loot and progression materials. That makes Skeletron more important than an optional reward boss.
Arena preparation
- Clear enough space around the Dungeon entrance.
- Build simple platforms for vertical movement.
- Bring enough healing for a sustained fight.
- Use a weapon that can keep damage uptime while moving.
Hands versus head
The hands create much of the early pressure. Decide whether removing them first makes the fight easier for your current weapon and movement style.
Movement discipline
Circle and change elevation instead of standing near the boss for long periods. The goal is to create repeatable safe attack windows rather than win through constant trading.
After Skeletron
Plan your Dungeon goals before entering so the newly opened area turns into useful progression rather than unfocused exploration.
Source: Official Terraria Wiki.