Por la tagmanĝo ŝi tranĉas legomojn per granda tranĉilo kaj metas salon sur ilin per granda kulero.

Por means for, so Por la tagmanĝo means for lunch or for the lunch meal.

In Esperanto, por often shows purpose or intended use. Here it tells us that the vegetable-cutting is being done for lunch.

• por = for
• la = the
• tagmanĝo = lunch

So this is not mainly about location or exact time. It is more like: as part of preparing lunch.

Esperanto very often forms new words by combining meaningful parts.

• tago = day
• manĝo = meal, eating
• tagmanĝo = the day-meal, meaning lunch

This kind of compound is extremely common in Esperanto. English does something similar with words like sunlight, toothbrush, or daydream.