Bow placement (or Contact Point) means how close or how far you are from the fingerboard or bridge. Beginners start with the bow placed about half way between the bridge and fingerboard. Sometimes the arm might not reach very far so a beginner plays closer to the fingerboard to prevent the right shoulder from pushing forward.
Perpendicular Path of Bow to each string
This is a very important topic for a beginner. You want to keep the bow in a perpendicular path to the string you are playing on and this is not as easy as it sounds! It can be hard to know if your bow is moving along a straight path. You can have someone look at you while you're playing and tell you but you'll probably need other ways in order to gain control over this. From the vantage point of the player, it is not obvious. Even looking in a mirror doesn't give too much feedback because it's hard to sense how you're actually doing it as you look into the mirror.
You will get the most consistent sound if your bow path is straight. If the bow goes crooked, the contact point will change and your bow will skate or slip towards the fingerboard or bridge. This skill has to be approached from many angles - no pun intended! You'll need to look, listen and use your sense of touch to learn this skill.
One fairly reliable way to learn how to keep a perpendicular bow path using your eyes, is to look at the side of the cello where the C bout meets the lower bout as you play. Keep watching so you can maintain the same distance between the bow and the spot you are comparing it to. This works pretty well if you're playing on the A string or the C string because they are closest to the side of the cello.
Go here to see a fun trick to learn the perpendicular bow path.
Bow Tilt
When you place your bow on the string, the stick needs to be tilted slightly towards your nose. Bow tilt is not having all the hairs flat. This allows the wrist to be lower than it would have to be if you played with flat hairs. If you always played with flat hairs, you'd put a considerable strain on your wrist.
Swells
Once you've been playing the cello for a while, you can experiment with playing close to the bridge. Swells are a great way to venture out from the middle lane. Imagine you are sinking into a sponge or some pudding. (video)