MSB & OB
These both terms are related to each other so gonna cover these in same para.
MSB
When price breaks a low or high then it shifted its structure from bullish to bearish and vice versa, called MSB or Market structure breakdown. A lot of terms are used for MSB like BOS , SB etc. These all have the same meaning.
OB
Order Block is usually used as a confluence to buy or sell an asset after MSB. The price normally tends to move to OB to fill the orders before moving to the otherside.