When a beatmap is first made, it is in the Pending or Work in Progress sections of the forum. These maps are referred to as "unranked". Unranked maps do not have a scoreboard and go to the Graveyard section of the forum after a period of inactivity. In order for a map to have a scoreboard and to be moved to the "Ranked" section of the forums, a map must go through the beatmap approval process. First it must get bubbled, and then ranked. Both MATs and BATs can bubble, but only BATs can rank a beatmap.
Before a beatmap can become ranked by a BAT, it must first get bubbled. In order for a beatmap to become ranked, it must first have 12 Star Priority (or SP). You can actively gain SP in two different ways. First, you can have others help mod your map. If you find their mod to be useful and give kudosu, you will gain 1 SP. You and/or others can also spend kudosu in order to "star" your map (A star costs 1 kudosu, and can be done through either the mapper's profile or through the map's thread). Once a beatmap has at least 12 SP, a MAT or BAT can bubble it if they feel it meets two conditions:
1. The beatmap meets the Ranking Criteria. Basically, this must mean that the map has the correct timing, has at least 2 difficulties (one of which is an Easy or a Normal), and is overall both mapped to the music and playable.
2. The beatmap meets overall quality standards that aren't part of the Ranking Criteria. For example, if a slider isn't very pretty, then a MAT or BAT would likely request for it to be made prettier before bubbling it.
Note: If your map has a bubble, then it will be popped if you update it. If the update wasn't drastic (ex. All you did was fix a slider), then just ask the MAT who bubbled it to rebubble it. Your map will gain 5 SP for a popped bubble icon.
Updating a Bubbled beatmap will cause this icon to replace it, it means it needs to get into bubbled status again, alternatively a BAT/MAT that disagrees with the judgement of the other staff member may place this icon as well. Some BATs place this icon on your map while modding it to prevent another BAT to look at it, and is very likely it will get ranked afterwards.
Once a map is bubbled, a BAT can rank it if the map meets the two criteria listed above. The reason two approval team members are required to check a map is because human beings make mistakes, and this greatly reduces the amount of times that a map makes it through the ranking system with a drastic error. If a map gets ranked but both approval team members who checked it missed a drastic error (ex. A 5 mb video is included in the .osz that isn't used), then it might get unranked.
People make mistakes. Therefore, if a map makes it through the approval system with a drastic error, then it might become temporarily unranked. If your map gets unranked, it's recommended to talk to the approval team member who unranked it in order to fix what was wrong so that your map may become reranked and enjoyed as quickly as possible.
Note: You'll sometimes be asked to change things you don't want to change. Sometimes you may be able to get away without changing it, but if half the Beatmap Approval Team agree that you should change it, then let go of your pride and just fix it.