Jesus gives these commands to encourage his disciples to pray continually. Alternate translation: "keep asking for what you need ... keep seeking what you need from God, and you will find it; keep knocking on the door"
To knock at a door is to hit it a few times to let a person inside the house know you are standing outside. It can also be translated using the way people in your culture show that they have arrived, such as "call out" or "cough" or "clap."