Something is sold if someone has bought it. When houses are sold, they usually put a "sold" sign at the front of the house that stays there for some time after the house has been sold.
A soldier is someone who joins the armed forces of their country. Soldiers defend their country from enemies, to keep the peace.
A solid is something in a state that is generally difficult to change the shape of. Most things that you can see around you are solids - your furniture, rocks, and the computer you are reading this on. The other two states that physical things can be in are liquid and gas. Both of these can change shape extremely easily. You can pour water very easily because it is a liquid, but you can't pour a brick.
You do something solo if you do it by yourself, alone. The person in the picture is giving a solo performance.
