The word lose is a verb, and it means to drop or miss something, or to not win a contest. The word lose is used quite commonly, and it occurs in a variety of idioms.
They will lose the game if they play badly. An airplane loses altitude when it starts to land.
She was losing the race, but came back to win a gold medal. I keep losing my keys.
The word loose is usually an adjective, and it is the opposite of tight. Rarely, it can be used as a verb, in which case it means to let go of something that is tied up (“Loose the dogs” ⇒ Release the dogs from their leashes). To describe the action of relieving the pressure of something that is tight, like a knot, one would usually use a form of loosen (“I tied my shoes too tightly, so I need to loosen my shoelaces.”)