When a house is burnt down, in English people say it is burnt up. When a boy swallows down the last morsel of his lunch, he eats it up. When an overhead door of a shop is pulled down and locked, it is locked up. What's up? Why does this English word seem to have such opposite meanings?
As an adverb, "up" has several meanings, and one of the two main different meanings refers to an erect or higher position; the other indicates completeness, which is less well known. In the first three sentences above, "up" means completely, and that explains the seemingly absurd usage of the word.
However, language sometimes is not logical but idiomatic. "Up" also has its just-so usage. One of its definitions in the Oxford English Dictionary is "used vaguely in a way similar to the use of down, round, over, across." So when somebody says, "She came up to me," he does not mean his position is necessarily higher than hers; rather, he refers to the direction. Just as when somebody says, "He went down the street," the street is not necessarily downhill. How can we non-native English speakers learn such idiomatic expressions? Memory. Don't try to rationalize why when I wind up my watch, it will go for a long time, but when I wind up this article, it will come to a stop. Just memorize the twofold meanings of this verbal phrase.