Caching is what a server will do when it determines that it might be requested for the same piece of information multiple times. In this case, when a request comes in, the server checks to see if it has the relevant information in the server cache. If it does, that is a cache hit. If it doesn't, that is a cache miss.
For the database server, sessions can mean several different things, but largely can be thought of as somebody logged into the database. For example, when you open SQL*Plus and log into Oracle, you have created an oracle session. Web Servers and App Servers will create sessions in the same way, though they can share sessions, or create multiple sessions, and so on.
Logical I/O and Physical I/O probably means the CPU or Disk usage by a partition. A Physical device (such as a CPU) can be tracked independently of a logical device (such as 45% of a CPU that is created as one logical instance, or several hard disks managed as a single logical disk). These are usage stats for those devices.
A Database Lock is what a database server will do when it needs to update or delete a record from the database. Locking a record simply prevents other sessions from accessing it in any way (or sometimes will allow read access - depends on how the database is setup). Generally, the lock on the record is released when the database has performed whatever operations were required on the record. For example, if I update my address information, the record becomes locked while the database updates the record. This prevents someone else from updating it at the same time, from deleting it, or other actions that could lead to database instability.