Marble is a natural stone, and is slightly porous. The marble we supply has been sealed at the factory and it is suitable for use indoors or out.
The sealer on the marble wears away over time with use / contact with acidity which breaks it down. Wine, coffee, and any spillages all contribute to this. If the marble is used outdoors and exposed, then the rain and outdoor conditions will wear away at the sealer faster. If the table is outdoors but under cover, then this will slow this down a bit.
As a rule of thumb the table will need to be resealed every 12 months to 3 years based on it's environment. The best way to test if it needs sealing again is with a glass of water. If you pour a little out and it pools, then the sealer is doing it's job. If it soaks in, it's time to re-seal.
When the sealer isn't working well any more, then any splillages etc will be soaked up by the marble. If this is red wine on a white marble table, then it can leave a mark.
You can reseal a table top yourself by purchasing a marble sealer from a stonemason / hardware store (e.g. Bunnings). It's usually available in a 1 litre tin. It can be applied directly with a soft rag, or by pouring a bit in the middle of the table and evening it out with a soft rag. It will start shiny until absorbed by the marble and then dull off.
In time, you may choose to completely re-finish the table. A stonemason as able to re-finish it by sanding the top and sealing again.