What is base metal what is it composed of?

It means the jeweller has used a non-precious metal. So it's not gold, silver or platinum but it could be any other metal. I think the most common base metal used by jewellery hobbyists would be copper because it's easy to work.

Because of the softness of pure (24k) Gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Typically 22k, 18k, 14k or 10k, contains higher percentages of copper, or other base metal alloys giving the gold more strength. Gold alloy with silver added appear greenish-yellow and is referred to as green gold. 18k gold containing 25% copper is found in antique and Russian jewelry. It has a distinct copper cast creating rose gold. White 18k gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silver in appearance.

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I hope this is helpful.

It could be darned near anything but usually its cheap steel.